Aquarium Medications and Treatments Part 1 | How They Work & Use

PAGE TWO; ANTIBIOTICS/ ANTIMICROBIALS
*Triple Sulfa, *Trimethoprim, *Tetracycline Hydrochloride
*Minocycline, *Quinine Sulfate,
*Oxytetracycline Hydrochloride, *Nitrofurazone
*Neomycin Sulfate, *Metronidazole
*Kanamycin Sulfate, *Erythromycin
*Isoniazid, *Penicillin & Ampicillin, *Naladixic Acid

PAGE THREE; PARASITE & CHEMICAL TREATMENTS;
*Trichlorfon
Internal Parasite Medications; *Piperazine, *Praziquantel, and *Levamisol
*Methylene Blue, *Mebromine
*Acriflavin Hydrochloride, *Malachite Green, *Formalin,
* Copper Sulfate, *Potassium Permanganate, *Hydrogen Peroxide

I also strongly urge reader to read the companion article:
A Healthy Aquarium; Disease Prevention
The above article is based on many years of hands on experienced and controlled experiments. Following the steps outlined will provide the reader the highest chance of success with their aquarium keeping endeavors!

This article (Aquarium Medications; How to Use), has information about different aquarium and pond treatments. A gram negative or gram positive application is given where they apply.
It is also noteworthy that not all human medications or veterinary are practical in aquarium or pond applications. A good example is Penicillin and Ampicillin (Please reference each of the sub articles above for more in depth information about different treatments).

Most bacteria which infect fish are gram-negative, including Aeromonas salmonicida, Flavobacterium columnare (Columnaris) Vibrio, and Pseudomonas species.
The major group of gram-positive bacteria which cause disease in fish are Streptococcus.

More about Gram Positive Streptococcus:
Aquarium Answers, Streptococcus, Eye Infections

When you have fish that are sick (bacterial, fungal, or parasites), you want to try and isolate the fish in a hospital aquarium whenever possible. Of course, this should not be done if the infection is systemic.
This is often seen with Ich or Gram positive infections such as Streptococcus.

Regular water changes before each treatment allows for a more effective treatment, especially when treated in the main display aquarium.
These water changes will lower dissolved organics, which could be a result of a previous treatment and not only absorb your latest medication dose (rendering it less effective), but can increase the toxic side effects of the treatment. The bottom line is that water changes need to be done before each treatment dose for the best results!

Sponge Filters work great in hospital aquariums. Remember to remove any carbon, as carbon will remove many medications. Also note that silicone in the aquarium will absorb Malachite Green, Methylene Blue, and Copper Sulfate (because they are a dye). Most corals crushed or similar, and ceramic decorations will also absorb medications.
Sponge Filter Resource: Premium Sponge Filters from AAP

Please see more on quarantine/hospital tanks in the Aquarium Disease Prevention Article:
Aquarium Disease Prevention; Hospital and Quarantine Aquariums

Correct calcium content (positively charged calcium ions) levels are important (newer research shows this more than ever), as calcium helps in healing and stress.
Without proper calcium levels healing may be difficult or impossible.

Make sure that when you do treat with antibiotics, you follow the recommended course including all recommended does and FULL treatment amounts (for all fish), especially when fish improve. This is a point that many aquarists do not follow.
Well meaning aquarists will purchase just enough medication for one or two doses when three or four are the minimum required. Then, the fish may improve somewhat and the aquarist assumes all is good, but the fish goes “downhill”.
Subsequent treatments with the same antibiotic are now not as effective due to the disease pathogen building up an immunity to this antibiotic.

I've had aquarists confront me stating “I thought you said this medication would work, the fish got better, but then they got sick again and now they're worse than before”.
When I dug deeper I found that only one or two treatments had been used and this is the common result when one treats with most antibiotics this way.
An exception is Triple Sulfa (or any Sulfa) which are not true antibiotics, rather Sulfas are antimicrobials. That said about Sulfas, they too are much more effective when the full course is followed.

Change Medications when Ineffective:

Since fish treatment is far from an exact science. It's important to try another antibiotic if after three treatments no results are seen (not 3 full treatment regimens).
Also if some results are seen, but the fish goes backwards as to remission of symptoms, I would recommend changing antibiotics/antimicrobials as there may be bacterial resistance. Weaker treatments such as Melafix or Pimafix may result this way, as while these can be good products for mild infections, these are generally poor for more serious infections.

Sometimes where there was effectiveness, but not a full cure that a second treatment regimen often at a stronger dose may be called for (keep in mind as an example that fin healing takes some time, so do not confuse this with suppression of the actual infection).
I do not recommend a 3rd treatment regimen, even if a different medication without a 7-10 day pause.

As I noted earlier, the majority of aquatic diseases are gram negative which make treatments with antibiotics such as Kanamycin or other gram negative antibiotics a good choice. This though is no guarantee that the disease pathogen will respond.
Sometimes combinations of Kanamycin and Nitrofurazone give a wide spectrum treatment, and for really difficult treatments, these can be combined with the Parasite/Fungal treatment "ParaGuard".
Other times you may have to try very different antibiotics such as Erythromycin or Metronidazole (Erythromycin is a good choice for usually aerobic gram positive eye infections.
Further information about eye infections:
Streptococcus, Eye Infections in Fish

Metronidazole is sometimes a better choice than Kanamycin or Minocycline for anaerobic (NOT aerobic) gram positive bacterial infections (especially internal) with the side benefit of mild effectiveness for internal and some external parasites.

Product Resources:
*SeaChem Kanamycin (Kanaplex)
*SeaChem Metronidazole
*AAP Furacyn PREMIUM Nitrofuarazone (vastly superior to non pharamceutical grade Furan 2)

The bottom line is to read the other pages of this article such as Aquarium Medications Two, antimicrobials, antibiotics (LINK) to UNDERSTAND the strengths and weaknesses of each antibiotic used.
You need to know whether aerobic, anaerobic, gram negative, or gram positive are what you will need for treatment.

For instance, after finding some success in treatment with subsequent backslide with a mild aerobic gram positive treatment such as Melafix, the next course may be a treatment with a stronger gram positive medication such as Tetracycline or Erythromycin. Even with Erythromycin, best use would be Erythromycin Phosphate as in AAP Myacin rather than the less effective and less broad spectrum Erythromycin sulfate popular sold by API and others.

Product Resources:
*AAP Myacin Premium Erythromycin
*Tetracycline from AAP

The previous example is just that, an example and a case of a less common gram positive infection.
The use of Erythromycin for example with no effectiveness would likely indicate a totally wrong medication choice, which would not be too unlikely since gram negative infections such as Aeromonas, Vibrio, Columnaris are much more common in aquariums.
Using Erythromycin as an example again, if it is effective for a suspected case of Columnaris, the facts are your fish did NOT have Columnaris since Columnaris is gram negative and Erythromycin is almost strictly gram positive (with the exception of nitrifying bacteria) and I have never seen Erythromycin ever be effective for a CONFIRMED case of Columnaris!!.

In the case of a gram negative infection that responding poorly to a treatment such as Triple Sulfa, sometimes using a combination treatment such as the before mentioned combination of Nitrofurazone (AAP Furacyn is best) and Kanamycin (especially if treating for Columnaris).

Also as noted earlier, sometimes combinations will allow for better effectiveness than each medication by itself (I would call this the cocktail effect which is employed in human treatments as well).
Combinations are especially effective when applied to Fish Baths.

See these articles for more about Fish Baths:
“Aquarium Disease Prevention; Fish Baths”
and
Fish Baths, Dips, Swabs

This principle also applies to chemical treatments for suspected protozoan infestations:

One point that I often use for both me and clients when microscopic evaluation is not possible is how the fish responded to a medication often is a determining factor in what the disease actually is.
Three diseases often get confused (for good reason), however treatment effectiveness is often a determining factor, see below;

• COSTIA: Formaldehyde


• VELVET: Copper Sulfate, Acriflavin, Methylene Blue


• SPOROZOA: Nitrofurazone (if anything)

Finally, often diseases are mis-identified, such as Fungus.
A good example is the use of Nitrofurazone which is found in different forms in many brands of Fungus treatment. HOWEVER this is not usually effective for true Fungus/Saprolegnia cases. If your treating for Fungus and switch to a product containing Nitrofurazone, likely your aquarium has Aeromonas and care should be taken to prevent this in the future.
See: Aquarium Aeromonas

VIDEO: AAP Professional Aquarium & Pond Medications
THE BEST AQUARIUM TREATMENTS ANYWHERE IN THE WORLD! Fresh Product found EXCLUSIVELY at AAP

Water Conditions when Medications Ineffective;

Often when multiple medications are used, often in strong mixed "cocktails", this can damage the aquarium (or pond) ecosystem/chemistry.
More Importantly often these problems pre-existed long before any treatments were started.

Checking ALL these parameters is a must, often before a treatment regimen, but especially if "nothing seems to work". Check the following parameters:

• Ammonia and/or Nitrites


• Nitrates


• pH


• KH


• GH


• Dissolved Oxygen below 5 ppm

My experience has often shown:

The usual causes of these are lack of established bio filtration, poorly treated tap water, or incorrect use of RO water.
Another cause I have observed over the years is the use of home water softeners for filling aquariums (which should never be done).

Please read these articles for some overview of these problems:

• Aquarium Nitrogen Cycle
  
• Aquarium Chemistry
  
• Use of RO or Water Softener in Aquarium

When NOT to use Medication:

Many readers of my articles both here at American Aquarium Information and Aquarium/Pond Answers might think I m very pro medication use based on how much information is about medications and their use.
HOWEVER in actual application over the years, I rarely use medications, in part based on practicing what I preach in article such as "Aquarium Disease Prevention".
Even when a fish might be a candidate for medication use, I still will attempt basic baths as a starting point or simply not treat in the case of an older or genetically weak fish where a medication might actually do more harm than good.

Here are some very simple points as to when NOT to medicate your fish (this includes organic remedies, as these too have side effects, despite the popularity and aura of "natural" around these treatments):

• As already noted; genetically weak or old/geriatric fish often do not benefit from medications/treatments, even in hospital or bath treatments. Sometime medications might actually fo more harm such as lowering red blood cell count or damaging kidneys in an already weak fish.


• After two attempts with different treatment regimens (generally no more than 10-14 days of combined treatments), the fish are still not responding, and assuming water parameters are good (including often forgotten mineral Cations), it is generally best to give your fish and aquarium a break of at least 10 days. This break INCLUDES; in tank, fish baths, and medicated foods.
  This failure could also constitute a virus (or even internal cancer) that is not treatable, a bacteria that is resistant, or simply a fish that has a genetic issue.


• Continued use of the same medication therapy or formula, even if fish do respond in a positive manner.
  Just as it is discouraged in humans, constant use of antibiotics can induce resistant bacterial strains and cause auto-immune or other issues within the fish' physiology.
  This include my own popular treatment combinations such as Kanamycin/Nitrofurazone or ParaGuard/Nitrofurazone/Metronidazole. Do NOT use these combinations over and over on the same fish or aquarium. Use another treatment in between or maybe nothing at all.
  
  Chemical treatments such as copper or malachite green do not count in the same way as do antibiotics

Back up Bio-Filtration:

As noted earlier, water changes before each dose are important to the effectiveness of treatment. The other aspect which may be important is the use of backup bio filtration. This is especially important when using medications such as Erythromycin which is hard on aerobic gram positive bacteria and nitrifying bacteria.

A SeaChem Resource: SeaChem Stability

Remove Carbon (& other chemical filter media):

Although most experienced keepers are aware of this, many “newbies” are not.
The use of activated carbon and most other absorbing media in filters can and will remove most aquarium treatment medications. Examples of absorbing media that should be removed include Chemi-Pure, The Poly Pad, Bio-Chem Zorb, & Purigen (this is NOT an exhaustive list).
If the carbon in your filter has not been changed in over 8 weeks, it likely will not be too effective at removing the medication from your aquarium or pond.

If you use filter cartridges, the answer is simple in most instances.
Simply & carefully slit the back of the cartridge and pour out the carbon so that the mechanical part of your filter cartridge can still be used.
You can also simply remove the cartridge, assuming you have other back up filtration without carbon or other chemical filter material. Then after medication is complete, rinse it well in TAP water to destroy bacteria and place it back in (unless it is old, then just add a new one). My personal preference still is to simply to slit the back and remove the carbon.
HOWEVER, NEVER simply turn off a filter when adding medication just because there is carbon or other chemical filter media inside this filter. This is the last thing you want to do and will only make toxic effects of any medication likely worse and the medication less effective due to deteriorating water conditions and lack of oxygen from less or no filters running.

Also note that some other types of chemical filter media such as Boyd’s Chemi-Pure or Filstar's Bio-Chem Zorb can also remove medications and should be temporarily discontinued during use of aquarium medication treatments.

Carbon or other similar chemical filter media can be replaced after treatment is finished or added to the filter temporarily for 2 hours prior to each subsequent treatment, then removed again.

SeaChem Purigen is another good product to use during treatment intervals or after treatment, more to the point, I STRONGLY suggests its use for a couple hours just prior to EACH treatment and then after treatment is complete!!
This product can be used with Carbon and compliments carbon in that Purigen removes many organic compounds carbon cannot remove which are toxic and often interfere with treatments.

The purpose of this is to remove any residual toxins that may have built up during each treatment dose which Purigen is uniquely able to perform. However, this is not required and is only an option to improve water conditions during treatment.

Mineral treatments (such as Regular Wonder Shells), buffers, or similar water chemistry additives can safely be used during aquarium medication treatments.

If bio filtration additives, such as SeaChem Stability are required due to ammonia or nitrite issues, these can be added about 50% of the time between each treatment dose.

Carbon Further References:
Carbon Use in Aquariums, Aquarium Answers
Filter Media Types, Aquarium Answers

Product Resources:
*Filstar Bio-Chem Zorb Chemical Filter Media
*SeaChem Purigen
*Wonder Shells; Unique Version only from AAP

Change Water:

Changing water immediately prior to each treatment or every 48 hours can improve effectiveness and lower the possible toxicity of treatment. For food soaks, once every 48 hours is also suggested.
A 20% + water change can cut back on organics in the water column that can often absorb many medications, thus lowering effectiveness.
However do NOT change water in the middle of a treatment dosing as this will dilute the medication and possibly render it ineffective. An example is if on day one and three a dosing is called for, the water change should NOT be on day two, rather on day three immediately before the new dosing

As well some medications break down after 24-48 hours and can leave mildly to moderately toxic chemicals behind which a water change prior to the next treatment can help lower or remove completely. As noted in the previous section, placing carbon or similar products such as SeaChem Purigen in a filter for an hour or two prior to a treatment can help remove ineffective or exhausted medications.
The use of small water changes and short term carbon, Purigen, etc. can also allow for longer use of many medications. This is especially important if using a medication such as Triple Sulfa to buffer malachite green medications and treat secondary infections of an Ich outbreak.

Tetracycline and Malachite Green are two examples of treatment medications that can be both more effective and less toxic if water changes (and/or carbon filtration) are performed immediately prior to a new or follow up treatment.

The use of the professional medicated slime coat water conditioner; AAP Res-Q is strongly suggested immediately following every water change. This product is superior to even Prime and vastly superior to Stress-Coat as per professional observations and use. Not only will AAP Res-Q remove chlorine from the water, it provides a medicated slime coat bandage that mixes well with the water and improves the effectiveness of about any medication regimen (whether fungal, parasite, or bacterial).
Resource: AAP Res-Q

Be aware that many chemical dyes such as Malachite Green can be at least partially removed by most water conditioners such as Prime, Start Right, AAP Shieldex & Res-Q, Amquel+, etc.
I suggest using the water conditioners after a water change, then waiting 15-30 minutes prior to adding a chemical dye medication treatment such as ParaGuard or AAP Super Ich Plus.

See: Aquarium Water Conditioner Use and Information.

However we want to be careful to not change TOO MUCH WATER, as this can dilute medications, in particular the use Medicated Wonder Shells or similar slow release medications.

Often a reason to change water is to prevent toxic build up of ammonia or nitrites in a treatment tank. However if your ammonia never exceeds .5 ppm your need for a water change for this reason is basically nil.
It is also noteworthy, that in a hospital tank in particular, if you have a fish that is not eating and/or being fed, normal respiration will add little to the nitrogen cycle.

As an example; I have made medication recommendations to many over the years, in particular non contract (non regular) service customers where by I set up a treatment regimen that in the end failed. In investigating, I found that they were changing 50% to 100% per day with no testing of ammonia to see if necessary. The result is the medications were not at therapeutic levels the proper amount of time to be effective.
The bottom line is stick to the 20% water change and only go higher if ammonia/nitrites are climbing. If using a slow release medication, NO water change should be used unless absolutely necessary due to dangerous ammonia/nitrite levels (of course having a seeded sponge filter to introduce during treatments is something I strongly recommend any serious aquarium keeper always have on hand).

Why I do not use Diagnostic Charts:

Readers of my Aquarium Information sites might also note that I do not use diagnostic charts.
The reason is simple. Most of the diagnostic charts that are posted on the internet or elsewhere are way over simplified and can mislead an average aquarist into miss-diagnosis.
This often results in frustration.

I will admit I've seen a few charts that have better flow than others, however even these do not take other variables into account. This often includes water conditions/parameters that can effect correct diagnosis of disease pathogen.
My point is a simple understanding of each medication and how it works is an important starting point.
Know when and when it wont work. Knowing more about the possible disease pathogens is far more important than a “one chart fits all” diagnostic chart.

For example: A fish with a Red Sore.
First one needs to determine whether this was caused by another fish.
If not one should find out water parameters and note tank conditions.
If this tank has poor circulation with a high bio load/DOC (dissolved organic compounds) it is likely caused by Aeromonas or similar since this can be an anaerobic bacteria that thrives in these kinds of water conditions.

Columnaris can also cause similar symptoms. It also is an opportunistic bacteria where improving electrolytes would also help (Calcium/Magnesium positive mineral ions).
With Aeromonas you would want to address the water conditions foremost before treatment otherwise your medication results may be poor, leading one to believe that they used the wrong medication when in fact the medication could not help the fish over come the perfect conditions (poor circulation)for this bacteria.

This is an important point. When medications do not work, it is often not a problem with the medications which is often assumed, but with water parameters and the choice of medication that was used for the problem.
For example you cannot use Neomycin for a severe Aeromonas infection also having poor water conditions and then say medications do not work.
In this case Aeromonas is a gram negative aerobic AND anaerobic bacteria that is much more resilient in low circulation, low oxygen water which is low in electrolytes.
It also requires a gram negative treatment such as Kanamycin as at least part of the treatment regimen. While Neomycin is an excellent fish treatment is primarily gram positive treatment.
Where Neomycin is best used is in the food to deliver medication to the gut where it likely will be more effective against anaerobic bacteria common to the gut (including some strains of Aeromonas within the gut).

This point also extends to parasitic diseases such as “Ich”. Medications such as Malachite Green are not only more effective in water with pH above 7.2 and a GH over 150 ppm. It's less stressful to the fish and the necessary electrolytes such as Calcium further buffer and aid in osmoregulation which further helps the fish resist an infestation.

Much more information on diseases referenced:
Aeromonas, Vibrio, Septicemia, Furunculosis bacterial infections
Columnaris, Fungus, Saprolegnia

To further summarize why diagnostic charts should be avoided is that they are simply people's opinions which are often vague.
Even true medical doctors (per my Mentor in fish treatment who was in the medical profession) often make educated guesses based on a lot more information on hand than the average aquarist has at their disposal (myself included).
Not to mention, medical doctors have a lot more training than most aquarists have.
This is important as several studies of online human diagnostic web sites have found the vast majority to be highly inaccurate when put to the test with real subjects where a true medical diagnosis has been confirmed.

How can we expect an aquatic diagnostic chart to be any more accurate when we have less tools and vastly less money for diagnosis at our disposal?

Effect of Water Parameters:

Make sure all your water parameters are where they should be. For example, if your ammonia levels are high and all the medication in the world will not help, make sure you have proper water circulation, dissolved oxygen, and low waste particulates. Otherwise your Redox Balance can be adversely affected (Redox Potential does not need to be worried about for the average aquarist). Adequate calcium levels (which can be partially measured by kH) are very important to proper osmotic function.

For more information see this article:
CALCIUM, KH, AND MAGNESIUM IN AQUARIUMS
More information on Redox: Redox Balance

• If any of the above parameters are out of sorts, you will have trouble affecting a cure, even with proper medication.



• Also important, proper efficient water changes (with proper electrolytes; salt magnesium, calcium) can do wonders without medication, so use ALL medications sparingly!!

Medication Misinformation:

I would like to address the common anecdotal comments made by many people about the harmful side effects of medications. While almost every medication has a side effect which can harm or kill a fish, only over use or misuse can result in harm. Even in human treatments, what can cure can also harm.

A comment often stated is the use of antibiotics will kill your nitrifying bacteria.
While this is true, it is ONLY true with new bio filters with many antibiotics (newly established aquariums). This only can happen with gram positive antibiotics or over use of others.
Although true nitrifying bacteria (Autotrophic bacteria) are gram negative, these bacteria are seriously impacted by gram positive rather than gram negative medications, possibly due to the bio films they produce (but the answer is not clear).
Reference: Aquarium Nitrogen Cycle; Nitrification.

The majority of aquarium infections (especially in marine aquaria) are gram negative. So the use of gram negative antibiotics such as Kanamycin (which is primarily gram negative) have a lower risk of a nitrifying bacteria die-off than when used properly/carefully in an established aquarium.
Some antibiotics are more mixed such as the Tetracycline families which are often equally divided such as Minocycline. However, Tetracycline Hydrochloride is primarily gram positive.

Another example is Malachite Green. This has been shown in studies to be carcinogenic when used in large quantities.
Yet in studies using this chemical at established therapeutic levels, Malachite Green has not shown to pose this danger.
Reference: Aquarium Medications 3; Malachite Green)

Here's another of piece medication misinformation or what I consider jumping to conclusions. In this area of two products by API; Melafix and Pimafix have recently fallen victim to aquarists and forums making assumptions based on poor scientific method.
I recommend reading page four of this article specifically dealing with these treatments:
ORGANIC TREATMENTS, Melafix, TTO, Pimafix, Usnea.

Basically I have read aquarists in forums stating Pimafix should not be used due to Clove Oil being in this product, however this is the same as stating a human should never use Tylenol as over use (especially with alcohol) can lead to liver failure.
If any problems have happened (I have never had an issue in 100s of correct uses) it is not likely used properly with normal water changes between doses (which I recommend with all medications).

Melafix has been blamed for the death of Labyrinth/Pencil fish, which I am not doubting the sources (such as Fish Lore) are reputable, but since I and others have conducted tests with Melafix at double dose with Labyrinth fish with no ill effects, I think we have to look scientifically at what the trigger may be (I explore this problem further on page four of the Medications article as noted earlier).

Another common problem is many will use only part of a treatment regimen, leaving out key synergistic combinations such as Nitrofurazone & Kanamycin (as found exclusively in AAP Spectrogram).
Other steps are also left out such as correcting water parameters (stating their ammonia & pH are OK when there are other issues) or failing to use topical treatments when called for such as use of AAP MethyBlu or Wound Control.
The common end result is failure or just partial success and then the aquarium keeper blaming the treatment for not working when in fact this was not the issue. Unfortunately many popular Facebook forums are guilty of giving only partial advice based on anecdotal results, furthering this problem.

Making False Assumptions when Medicating:

This is similar to the section above, however here I'm referring to aquarium and pond keepers use of medications vs. the previous section dealing with anecdotal advice.

A good example is the use of Erythromycin or Melafix for treating Columnaris which is gram negative and then assuming your use of Erythromycin cured this ailment when laboratory tests prove this impossible (although Erythromycin Phosphate is more wider spectrum than the commonly used Erythromycin Sulfate used in products such as Mardel Maracyn & AAP Erythromycin).
What likely happened is some other part of your tank maintenance procedure took care of this opportunistic infection OR quite simply your fish did not even have Columnaris, rather Streptococcus or FNT Disease or other gram positive diseases.

More information on: Streptococcus & FNT Disease

Changing water or otherwise improving water conditions can often allow a fish overcome an opportunistic infection (such as Aeromonas). This easily allows the aquarium keeper to assume that a treatment cured his/her fish.

My point is that this hobby is often driven by anecdotal assumptions that I would like readers to avoid by thinking more scientifically.
A controlled study of medication use would include control groups, exact maintenance/water change procedures and more to keep different medication tests accurate.
Obviously the aquarium or pond keeper when confronted with sick fish does not have time or multiple aquariums/ponds to perform these tests.

However by changing water before treatments (even before each additional treatment), keeping water parameters where they should be for the fish kept, and knowing what each treatment method can and cannot effect, as well as proven/known effective treatments for bacterial & parasitic pathogens can allow an aquarist to know when an infection treatment is the best possible to utilize.
So please understand the importance of healthy water parameters, and knowing what a medication can and cannot due rather than listening to or making your own anecdotal assumptions.

Best Used By or Expiration Dates for Medications & Fish Foods:

Despite common belief, 95% of medications AND Fish Foods are still quite effective well past their expiration/best used by dates printed on packages (generally an antibiotic is at least 90% + effective 6-12 months after said date).
The exception is Tetracycline Hydrochloride which can become toxic past its expiration date, although even then it takes well over a year for this to happen (generally years).

Many organic treatments such as Melafix can last for years past their expiration dates (I used some Melafix that was two years past its date with positive and measured results).
Most Chemical Treatments such as Cupramine (Copper based) or Quick Cure (Malachite Green, Formalin) have no practical expiration date. By Practical, I mean chances are the medication will out last you and your fish.
At most the copper in Cupramine might precipitate out of the solution, but often a good skaking can remix the product.

These “dates” are in part made by regulations (often under false assumptions) as well as companies which manufacture these products who know when you discard a medication, you will likely purchase a replacement.
In fact I've used many medications past these "best used or expiration dates" including products such as AAP Marex containing Chloroquine Phosphate which gets less effective quicker than many medications, yet while results were slightly reduced, they still worked quite well. The problem of late is the internet is now flooded with anecdotal forums where members have little practical experience and do not allow for any mentoring such as I have had with Dr. Herzog many years back (he was a Harvard grad Endocrinologist who was also very active in the fish hobby and added a lot to my education along with the scientists at Aquatronics many years back too).

Either way, neither reason is based on good science which includes many studies, as well as my own multiple controlled tests over the years.

If you doubt this, a quick search on the internet, including medical sites will prove this correct. Don't believe what you read on Yahoo Answers or similar anecdotal sites.

Reference:
*The Big Myth, Food Expiration Dates

Gram Negative/ Gram Positive Bacteria

Here are a few terms:

*Gram-negative bacteria are pathogenic, as they can cause disease in a host organism and have a thin cell wall. Gram negative bacterial infections are much more common in fish, especially marine. These bacteria that will NOT retain a crystal violet dye during the Gram stain process.

Please click picture to enlarge

Here are some Gram Negative Bacteria:

-Aeromonas (anaerobic)
-Furunculosis; Aeromonas-salmonicida (anaerobic)
-Vibrio (anaerobic)
-Flavobacterium (Columnaris) (aerobic)
-Pseudomonas (aerobic)
-Streptobacillus (anaerobic)
-Salmonella (anaerobic)

Aerobic Chemolithotrophic Bacteria:
Nitrifying bacteria
-Nitrobacteraceae (aerobic)
-Nitrobacter (aerobic)
-Nitrococcus (aerobic)
-Nitrospina (aerobic)
-Nitrospira (aerobic)
-Nitrosococcus (aerobic)
-Nitrosolobus (aerobic)
-Nitrosomonas (aerobic)

Anaerobic Chemotrophic Bacteria
-Erythrobacter (anaerobic)

- Cyanobacteria (Oxygenic Phototrophic)

Gram-positive bacteria do not always cause a disease in the host organism and have a very thick cell wall.
Gram positive infections are less common in fish (or aquaria in general). These are the bacteria that retain a crystal violet dye during the Gram stain process.

Here are some Gram Positive Bacteria:

-Mycobacterium (aquatic tuberculosis)
-Streptococcus (Pyogenic Hemolytic Streptococci, Oral Streptococci,Enterococci, Lactic Acid Streptococci, Anaerobic Streptococci)
-Pseudonocardia (anaerobic)
-Staphylococcus (aerobic)

Of the above bacteria noted, Flavobacterium (Columnaris), Vibrio, Aeromonas, & Pseudomonas cause the vast majority of diseases in aquariums/ponds AND these are gram negative. It should also be noted that Aeromonas and Vibrio are generally anaerobic, so keeping a well circulated/oxygenated aquarium or pond is important for their prevention and treatment.
Mycobacterium is an occasional problem that is gram positive.

I have cited much of the information for this section from this source:
Classification of Bacteria

Contraindications (& Medication Mixing):

What are "Contraindication"?
From Wikipedia: https://en.wikipedia.org/wiki/Contraindication
"In medicine, a contraindication is a condition or factor that serves as a reason to withhold a certain medical treatment
Some contraindications are absolute, meaning that there are no reasonable circumstances for undertaking a course of action
Other contraindications are relative, meaning that the patient is at higher risk of complications"

With this in mind I have added a "Contradindications" section under many explanations of each medication treatment on page 2 and page 3 of this article.

Knowing some of these contraindications will help determine whether it is advisable or at all safe to mix certain medications or chemical treatments. As well, the mix of medication and volume depends on the disease you are trying to treat and the inhabitants of your tank.

Here are some examples of medications that can or cannot be mixed:
Tetracycline, NEVER mix Tetracycline with any other medication. The reason for this is that it is a very harsh treatment and lowers the red blood cell count in your aquarium fish.
Potassium Permanganate can be mixed with salt ONLY, and is used to aid in the treatment of external parasites such as Anchor Worms.
Triple Sulfa can be mixed with aAP Super Ich Plus or Paraguard as it aids with secondary infections and lowers the ricks of infections caused by the irritation of malachite green with certain sensitive fish such as Cory Catfish or Clown Loaches.
Paraguard, Furan 2, and Kanaplex can be mixed and this combination aids in the treatment of a wider spectrum of diseases.

For those looking for ACCURATE information about stepping up to a higher level of aquarium or pond fish care, this article is a MUST READ:

Ultraviolet Sterilizer Use; Facts & Information
The most RESEARCHED, & IN-DEPTH article on the subject of UV Sterilizer, Clarifier Use in Aquariums or Ponds ANYWHERE.
Based on over 34 years professional use too!!

Related/ Recommended Products:

For Aquarium/Pond treatment products to help with treatment & prevention many aquatic diseases, as well conditioners, API Test Kits & more.

Such as Melafix & other Aquarium Pharmaceuticals Treatments.

For all your UV-C Replacement bulb needs for Ultraviolet Sterilizers, Clarifiers, and even Purifiers, see this web page:
Germicidal Bulbs

SeaChem
SeaChem Aquarium and Pond Treatments, Conditioners, etc;
The very best Aquatic Products in their category, bar none.
SeaChem products include; SeaChem Kanaplex & Cupramine (an excellent replacement for Aquarisol)

AcroPower Amino Acids for SPS Corals
Two Little Fishies new formula AcroPower supplies essential amino acids that corals need to build their skeletal architecture

Aquarium Silicone
Professional Grade Aquarium Silicone

Reverse Osmosis Filter System with TDS Meter by TMC
The PREMIUM RO Filter system what requires no added water pressure booster pumps in most applications, unlike many others.

TMC Vecton & Advantage PREMIUM UV Sterilizers
There is NO Better UV for your aquarium or pond;
Maximum dwell time and performance that will improve all fish disease resistance UNLIKE most now sold which are little more than water clarification devices (such as those sold on eBay & Amazon)!!!

Hydor Smart Wave Electronic Controller
Adds natural currents that promote healthy reef aquariums.

Wonder Shells, Regular & Medicated
Unique Version sold ONLY at American Aquarium Products.
Excellent for disease prevention!!!

Aqua Master Ultra Premium Koi Food