I will discus two primary algae problems; green water (or free floating algae) and attached algae such as common string algae (also known as hair algae, blanket algae or Blanket weed).
First a brief description: Algae are plants that vary from small and single-celled to complex multi-cellular species that use photosynthesis to produce nutrients they need from sunlight and other sources. Even though they are small in size, they are very strong plants. Similar to most plants, algae require organic and inorganic nutrients, light, carbon dioxide (oxygen at night), water and heat.
First of all optimum water chemistry is important to control both types of algae.
A fully "cycled" pond with ammonia and nitrites at 0, nitrates under 50 ppm, and phosphates near 0.
I would also like to note that I do NOT recommend the use of chemical pond algae killers as these often can exasperate the problem in two ways. One is that these chemical do not remove algae, they kill it where it falls to the bottom and adds to decaying organic mulm that then adds more nitrates and nitric acid to your pond which in turn spurs the growth of even more algae. The other problem is related and that is as the dead algae decays it depletes oxygen which can be deadly to the fish and spur the growth of toxic Blue Green Algae (Cyanobacteria). The nitric acid production can also lead to dangerous pH crashes if the proper KH is not maintained.
Green water is basically single cell algae that is usually too fine to be filtered out by most common pond filters (sometimes Diatom filters can achieve this).
Control of green water may involve several parameters and methods or sometimes just one or two.
Here are parameters and methods worthy of note:
Decay and Bio Load;
If you have a lot of decaying matter in your pond (usually on the pond bottom, however this can also be in a poorly maintained or designed filter as well), a high bio load (too many fish, fish waste, uneaten food, etc.), these will add to Nitrogenous waste (such as nitrates) which will provide nutrients for algae growth.
Lowering your fish count, improving the quality of your Fish food (to lower fish waste due to lower food digestibility due to poor ingredients), and most important lowering the amount of decaying matter in your pond will improve this factor for algae control. Also as per the nitrogen cycle make sure you have a functioning Bio system in your pond (via filtration, veggie filters, etc.).
Having a layer of porous gravel/rock (such as volcanic rock) on the bottom can often help with bio breakdown of organic waste
Often I would have a customer call my pond service after purchasing a UV Sterilizer claiming it did not work well and when I arrived at their pond I would find not real filter, only a pump running a UV Sterilizer and a few plants. The fact is while I recommend a UV Sterilizer/Clarifier, it cannot be your only tool in green water control; good filtration (and pre-filtration for the UV Clarifier) IS ESSENTIAL!
Probably the king of green water control, A properly installed UV Sterilizer with a pond turnover of at least once every three hours and a flow rate through the UV of no faster than 40-50 gph per watt is very effective for the control of green water.
Better designed UVs can effectively control green algae at higher flow rates of 60 gph plus; a good example is the TMC Advantage PRO Pond UV Clarifiers.
In fact of the dozens of brands UV Clarifiers I have use over the last three decades for pond green water control, no UV Sterilizer has come close to the TMC Pro Pond line when efficiency, flow, kill rate, durability, and even price is factored in; and this includes over others I still consider excellent such as the Terminator or my own custom UV!
The use of a properly installed UV will also aid in maintain a healthy redox and even GH levels as photosynthesing Green Algae will be lowered substantially.
*It is noteworthy that your UV-C Bulb/Lamp must be changed every six months in warmer climates and once per "season" in cooler climates for your UV to remain effective. Another potential issue I have observed is many pond keepers will rely 99% for their pond clarity on the UV Sterilizer, ignoring good filtration (especially a bog/veggie filter) and many other important green water prevention technics. The result is a UV Sterilizer that works one season, but not the next, even with a new UV Bulb due to a bio load that eventually exceeds the ability of the UV Sterilizer.
Veggie Filter (plant filter);
a properly constructed Veggie/Bog Plant filter can remove many nutrients that algae need and are one of my more successful methods for pond green water and algae control (even without UV Sterilization). Another way a Veggie Filter helps is through acid production that counter acts the photosynthesis of algae. The only negative with these is time, as a Veggie filter often takes one full summer to reach full potential, so immediate results are usually not realized (but at least you will be ready for the next summer!)
Please note that floating Lillies do NOT count as a Veggie filter, as these remove very few nutrients. The primary advantage of floating plants would be to shade the pond.
Please read this article for more information and methods for veggies filters: Pond Veggie (plant) filters The first four above are probably the most important for green water management/control.
The next 3 (5-7) are next in priority in my experience
Use of a trellis to shade your pond can cut sunlight (a major ingredient of green water) by 25% or even more (depending upon the amount of direct and indirect sunlight blocked).
Floating plants such as Lilies and Water Hyacinth can block some sunlight and also provide some help with nutrient removal as well.
Algone is reasonably effective for ponds under 2500 gallons, especially when used as part of a green water control regimen (not the sole method).
Algone works with enzymes and nitrogen fixing microrganisms that help control waste and most importantly; Nitrates, thus helping to starve free floating algae of one of their most important nutrients.
this is a so-so method in my opinion for green water control, however this can work or aid in green water control especially in milder cases or in conjunction with other methods and parameter controls. Barley Straw products work best when combined with other methods or products. Examples include Barley with Peat and Pond Keeper with Barley Straw which is barley combined with Heterotrophic Bacteria.
Pellets or even extract are more effective than plain straw due to the higher concentration of Iodine which has been shown to be the effective ingredient in this method.
this may seem silly to some but I have controlled green water (not severe cases though) with what are referred to as “algae magnets”. Technically there is nothing special about these magnets, as really any magnet of about 3 x 1 inch or larger will work. These need to be placed in a high flow rate area, usually the filter or around water lines.
How these magnets work is they simply attract iron that the excessive free floating algae needs for photosynthesis.
My recommendation for magnets is generally to use them as part of a green water eradication plan, not the whole plan.
Water Changes/ Auto Water Changer (Spring fed pond);
This is of course an effective immediate answer to “pea soup green” ponds that usually are not fully established and have no UV Sterilization (albeit a temporary solution). If done carefully, even large water changes of 50% can be performed provided chlorine is removed and water is added very slowly to prevent temperature shock. Make sure and try and vacuum out any mulm that may have built up on the bottom as well so as to make for a more efficient and effective water change.
A pond owner can also set up a small overflow and inflow for constant water changes which can also be effective for clear pond water (& healthy Redox as well). In some cases a Spring fed pond is possible, which makes for much easier green water control. However your own homemade “spring fed” pond can still be achieved by the overflow, which is more easily achieved if the “fresh” water originates from a well so that chlorine removal is not necessary. Even if city water is used, very small amounts of chlorine are usually not a problem and the use of products such as Wonder Shells can remove any small amounts of chlorine present (and improve water quality in other ways as well).
these can also remove nutrients (although not nearly as well as a veggie filter since there is not the root structure and anaerobic filtration that a well constructed Veggie/Plant filter provides), however the results can be more immediate since any healthy plant removes at least some nitrates and phosphates (which lead to green water production), also floating plants can also block sunlight which is another ingredient in Green Water algae production.
Here are few fast growing floating pond plants that can help with Green Water Control:
• Water Lettuce
• Anacharis (Elodea)
• Water Hyacinth
Please see this website (not mine) for more about some of these plants and where to get them: Floating Pond Plants
As noted above, a GH between 100 to 300 and a KH between 50 to 150 are can play a role in green water control (albeit a more minor role).
This effects Electrolyte levels and Redox Potential as well as pH. Water changes, ‘flow through’ circulation, and products such as Wonder Shells can also help (avoid use of Plaster of Paris or Lime). If your GH is low this can be a problem during the hours of the day when photosynthesis is high, even with an adequate KH. It is possible due to the production of carbonates by plants and algae without adequate water hardness (GH). In a pond with high algae growth this can especially become a problem due to increased photosynthesis. So increased KH and especially GH are very important for ponds with problem algae.
Some inert Blue dyes can be used like a sun block, these work to prevent light penetration into the pond which slows or inhibits algae growth. Methylene Blue may be used in a pinch, however even though it works even better than ordinary blue dyes for algae growth (due many of Methylene Blues medicinal properties) it also will destroy beneficial nitrifying bacteria and is very harsh on aquatic plants with long term use.
Copper Sulfate is effective in controlling green algae, however it has side effects in that it is not good for pond plants and is difficult to remove from ponds.
I do not recommend these as these are only a quick fix that often make the situation worse in the long term. As algae are killed, they then die and add to the bio load which will then add more nutrients and pollutants to the water column which will further exasperate the algae problem. BLANKET WEED ALGAE (Long String Algae);
Veggie Filter (plant filter);
these remove many nutrients that algae need. I have not had quite as good of success with blanket weed control as with green water control, however they still aid substantially when used in conjunction with other sound pond maintenance procedures.
The Veggie/Bog Plant Filter is probably the best "weapon" against hair algae!
I have found this more effective for control of blanket weed than green water. Beer has also been used for blanket weed control.
Barley pellets or even extract are more effective than straw due to the higher concentration of Iodine which has been shown to be the effective ingredient in this method.
Decay and Bio Load;
Remove as much "Muck" from the bottom of the pond as possible.
The idea is to remove as much nutrient producing decay as possible so as to cut off a major nutrient source for blanket weed growth.
In extreme growth outbreaks, lowering the water so as to scoop out and/or pressure wash the bottom or other areas (such as waterfall tiers) may be necessary.
this kind of goes without saying however since a pond is a living organism unlike a swimming pool, algae is going to be present in some form no matter what you do and removal by hand or implement is to be expected.
As noted for Green Water Control, Algone is reasonably effective for ponds under 2500 gallons, especially when used as part of a multi-part algae control regimen. That said, while Algone is a useful tool here as well, it plays a more minor role in controlling Blanket Weed/ Hair Algae than it does with green water control.
Algone works with enzymes and nitrogen fixing microrganisms that help control waste and most importantly; Nitrates, thus helping to starve algae of one of their important nutrients.
Flow through filtration;
adding an over flow to remove water while a small amount of water is constantly added will control nutrient levels and aid in Redox and mineral replenishment (depending on the water parameters of the new added water).
Again as noted above, a GH between 100 to 300 and a KH between 50 to 150 is helpful.
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