A general principle (not a rule) of about 50-200 gph per 100 gallons can work for this (A lower proportional gph for large ponds, a higher proportional gph for small ponds). This can be achieved with more than one water pump.

A large aeration device (air stone) coupled to an Air Pump can also aid in circulation and are especially useful for “lifting” (vertical circulation) water from the bottom of a pond when properly placed. Often an air stone/air pump combination is better than water pumps of greater gph for the aspect of vertical circulation (keep in mind that an air stone aerates the water by breaking surface tension, not by the bubbles in the water; the more surface agitation, the better oxygen exchange). A large double air pump such as a Fusion 700, SunSun YT-302, Whisper 800, or Maxima with two air stones can circulate at a rate up to 500 gph. A way to check this is to place the air stone down a narrow PVC tube or aquarium lift tube, then place the top of the tube just above the water level and measure the rate the water fills up a gallon container (one gallon in 15 seconds= 240 gph).

With any pump (Water or Air), at least some form of circulation should be provided 24/7 so as to provide both oxygen (& Redox Balance), but also maintain a healthy bio filter (this also means a pump connected to a filter should run 24/7). Most pumps properly placed do not disturb fish, despite a few rumors to the contrary.

General Pond Pump Recommendations:
For cross circulation (& lifting water to water features), a water pump is the primary choice.
To start with the Rio HF Pump line is an excellent line of pumps for most ponds from 300 to even 6000 gallons from my experience as these pumps have top notch head pressure, they are reliable and have flow rates up to almost 2000 gph.
What I have found is that even with larger ponds having multiple reliable and powerful pumps such as the Rio 32HF, I have more redundancy, more flexibility, and simply less "disasters" with a single "super large" pump pond. As an example, I set up a 3000 gallon pond with three 2000 gph pumps feeding a fountain, veggie filter, two UV Sterilizers, and a DIY barrel filter and then a waterfall with good aesthetic flow.



To protect your pump from large debris (that can damage the impeller and impeller housing), you can also place your pump in a DIY bucket (or box, etc.) pre-filter. Besides protection from debris, this added pre-filtration will provide biological filtration.
This is especially useful for pumps added solely to run UV Sterilizers.
Use a bucket, box, or similar container large enough to house the pump; then add Volcanic Rock about ½” in diameter or larger, an outlet tube, then either seal the container with holes in the top OR simply leave the bucket, box, or similar container open with only the rocks/gravel exposed.
To the left is a diagram of how this would basically work:
Click picture to enlarge


For small to medium ponds (100–800 gallons): the SunSun JQB-3500, Via Aqua 3300 Pump, or Rio 1700 all are proven efficient and reliable, as well as reasonably economical pumps (the Rio 1700 has an available water Fountain Kit).
All three have sealed electromagnetic motors and ceramic shafts.
The lighter duty Via Aqua 306 or SunSun JP-066 can also be used in ponds, however I recommend using either the Filter version or making a pre-filter bucket to protect it due to the standard 306/JP-066 not having a pre-filter (described below, which is a good idea for any pump unless a Hydro Pond Pre-Filter is used, which cannot be attached to a 306/JP-066).
The most of these pumps such as the Via Aqua 3300 can easily be mated to a Fountain Head with built in diverter then connected to a Filter (such as a small Pressurized Filter), and a UV Sterilizer (such as the TMC 15 Watt Pond Advantage PRO UV) for up to a 500 gallon pond

For larger ponds (500 – 2500 gallons); the Via Aqua 8000 Pump is hard to beat for value; quite honestly I do not feel there is any better pond pump than these for under $150. In fact this pump at under $100 compares in water volume and quality of pump to may pond pumps sold elsewhere for $250 and more.

For even more head pressure for high pond features (such as a water fall or spitter) that require lifts of much more than 6 feet the Rio HF Water Pump line is without comparison with their Vortex Rotor Blade (which can lift up to 14 feet), especially for the price.

The Via Aqua Multi-Purpose Pond Pump can also be used for medium to large ponds (generally over 1000 gallons), especially when its multi-purpose versatility is needed such as running a larger fountain, powering a pressurized pond filter, and circulating water to a Veggie Filter all with one powerful pump

For Very large ponds; often the best way to go is with a heavy duty submersible pump with a directional vortex impeller or a pool/spa style pump that is designed for the harsh pond environment (some pumps cannot stand up to the debris and other elements of pond water circulation).
The Via Aqua Multi-Purpose Pond Pump at 3828 gph with a Directional vortex impeller (probably the best value in heavy duty pond pumps) or the SeaHorse line of pond pumps offers flow rates up to 4900 gph and are very well made reliable pumps that usually handle the harshest of conditions.

For these heavy duty 1/2 to 3/4 hp pumps or the Via Aqua Multi-Purpose Pumps I have generally used 1.5 inch PVC pipe and then split the return pipe suing PVC ‘Ts’ and valves to control flow through other pond filters such as the Clear Stream Pressurized Filter and a UV Sterilizer. The purpose for this is to run multiple filters, UVs and/or to have a slower flow through these devices which generally work much better than at the high flow rate provide by this type of pump.
Using PVC 'T' Diverters also allows for the use of these large pumps for smaller ponds under 1000 gallons where multiple features and/or high head pressure features are being run.

Even with a large pump like the SeaHorse, I still recommend another smaller one for redundancy in case of failure, and often for large ponds I simply use several pumps like the Rio 26H

For further Water Pump Specifications & Tips, please see: Pond, Aquarium, Fountain Water Pump Tips

Dissolved oxygen is VERY important in ponds (partly for the Redox Potential/Balance). All oxygen is exchanged at the surface, so good surface agitation over the entire pond is best. Waterfalls, fountains, air stones all can achieve this.
Low dissolved oxygen levels and a low kH & GH (below 50/100 ppm respectively) affect your fish’ ability to perform osmoregulation & Redox Balance will be poor. If these parameters are poor, this will allow for much more opportunistic infections. The fountain head in the picture to the left (click to enlarge) is an excellent way to aerate.
Some examples are available here: “Rio 1700 and Fountain Head” and “Via Aqua 2600 and Fountain Head”.

Make sure in winter months to de-ice and circulate water to the surface for optimum fish health.
For further information about GH/KH, please see this article: CALCIUM, KH, GH, AND MAGNESIUM IN AQUARIUMS & PONDS

For further information about Redox Potential, please see this article: REDOX POTENTIAL IN AQUARIUMS & PONDS; How it Relates to Proper Aquatic Health

[2] GOOD FILTRATION.

Good filtration, especially biological is essential to maintain a healthy pond with the only exception being with a “flow through/spring fed” pond. Understanding the Nitrogen Cycle (which is the same in a pond as in an aquarium with the added input of debris from trees, wind borne, and sometimes wild/domestic animals adding to this strain on the bio system.
More than one filter is always best when possible and this includes DIY natural plant/veggie/bog filters (which add to the aesthetics and are an enjoyable DIY project in my opinion.
Multiple types of filtration in combination are also excellent, such as these to name a few: a pressurized filter, bio falls combination, sponge filter, fluidized, barrel, and/or veggie filter,

The picture to the above/left shows a Via Aqua Multi-Purpose Pond Pump embedded in loose volcanic rock (for added pre-filtration) connected to a Pressurized Pond Filter and then to a High Output Pond UV Sterilizer/Clarifier.
This picture displays how a large pump (as shown with the high flow VA Pump) can be utilized for more than one function, including multistage filtration, UV Sterilization, a fountain and even can be diverted for watering a Veggie Filter.
Please note that a control valve may be needed on both sides of the diversion ‘T’, otherwise the water will follow the “path of least resistance”, put another way if your water flow is too slow through the UV Sterilizer/Filter side of this example, it may be that the return is too high above the other return and simply lowering the return will solve this problem. As well after splitting a return line, the lines should not be combined later in the return. Please read the UV Sterilizations section for more on this subject.

I do not recommend the old style Becket or Pond Master Submersible filters; they clog easily, are hard to service, and are not efficient filters.
However the Pondmaster 1000/2000 can be vastly improved by unscrewing the screws in each corner and adding biological filter media to this area under the tray (as the Pondmaster 1000 only utilizes two pads that only perform mechanical, some chemical , and no essential bio filtration).
Volcanic Rock is excellent for this or better yet would be SeaChem Matrix (which can be placed in a nylon filter bag for smaller filters). Both of these would add considerably to bio filtration of your pond by adding to your filter.
In fact Pond Matrix provides the best internal macroporous surface area of ANY product and is an excellent compliment to a pressurized filter, waterfall filter, drum, or large sand/bead filters such as the AquaBead. These macropores are ideally sized for the support of nitrifying and de-nitrifying bacteria. This allows Pond Matrix, unlike other forms of biomedia, to remove nitrate along with ammonia and nitrite, simultaneously and in the same filter and rarely if ever needs to be changed if properly cared for. Pond Matrix also makes an excellent support for root development of aquatic plants and may be used for Pond Veggie/Bog Filters or in pots

I also suggest replacing the black carbon impregnated pad in the Pondmaster with a cut to fit Poly Pad and add carbon or Zeolite underneath the tray (also in a filter bag).

I prefer pressurized pond filters such as the "Clear Stream", Via Aqua EF-6000 UVC Pressurized or others for their efficiency and ease of hiding in the ground.
These filters include the popular Tetra, Lifeguard Pressurized, and Cyprio, which are excellent too, but tend to be over priced.
Pressurized filters can be buried in the ground or place behind a water feature easily. Pressurized pond filters are very good bio filters especially when price is considered and these are also excellent mechanical filters.
However many pressurized pond filters are sold with attached/built in UV Sterilizers which when the flow rate/dwell time is considered usually provides little green water control sterilization and absolutely NO level one sterilization. A separate UV such as the TMC Pond Pro Advantage is vastly more effective when plumbed properly.
(See UV Sterilization; Facts & Information

Do NOT confuse an aquarium canister filter with a pressurized pond filter, where as although a pressurized pond filter can be used for an aquarium, the use of an aquarium canister filter for a pond will result in failure since most aquarium canister filters have top mounted motors that will loose their siphon in most pond applications, as well an aquarium canister filter is not nearly as rugged in design for the pond environment (I had a couple of service calls many years back where customers attempted to utilize an aquarium canister filter for their pond and results were disastrous at best!).

Small or Patio Pond (under 250-500 gallons depending upon fish stocking and natural debris fallout);

For small ponds and patio ponds (ponds built in above ground "Rubber Made" or other similar containers such as converted horse waterers), large sponge filters are excellent bio filters and reasonable mechanical filters. They are inexpensive, easy to service, and simple to attach with either a power head pump or and air stone.
There now is a specific Pond Sponge Filter called the Hydro-Pond, with two models; one air driven, one pump driven. They are useful up to 1500 gallons and more can be used for larger ponds or in combination with other filters. The Hydro Pond IV also makes an excellent pre filter for pond pumps before water is circulated to another filter such as a pressurized filter.

For "patio ponds", these filters can perform very well since the only weakness is inability to handle large debris, but with patio ponds this is rarely a problem. In fact one client of mine with a 150 gallon patio pond had a small Becket pond filter he purchased at Home Depot that did not keep his pond clean, however when he switched to a Hydro Pond #2 his water while still somewhat green was much more clean and the fish could be readily seen.
I highly recommend the Hydro Pond filters for small pond owners on a budget, owners of patio ponds, or just to add redundancy to existing pond filters (often is larger ponds)

A top notch (& unique) filter for ponds up to 500 gallons would be a TMC V2 1500 Fluidized Filter (please read more in "Pond Filters of Note" a few paragraphs down).


The Via Aqua BH-2000AD; Small Pond Pump/Filter, LifeGuard All in One and Complete Wet/Dry Filter System are other reasonable filter choices for small ponds under 500 gallons. These filters along with the Hydro-Pond Sponge and Fluidized are an excellent compliment to a Veggie Filter (which I recommend for any pond).

With the Via Aqua BH-2000AD (pictured to the left) you can easily attach a UV Sterilizer to the “diverter” valve (just above the filter and below the fountain head) for improved water clarity. A UV Sterilizer can also be added to the diverter valve of the Lifeguard all in One or the Pondmaster 1000 filter as well.
See the UV Sterilizer section further in this article for a diagram.

Medium Pond (250-2000 gallons depending upon fish stocking and natural debris fallout);

Bio falls such as the Savio Livingponds Filters are also excellent pond filters for medium and many larger ponds.
These filters can also be DIY (Do it yourself) or you can start with a basic Bio Falls “skeleton” filter (pictured to the left) and then add bio filter media (such as Volcanic rock) and place a Polyester Media on top.
However if you have purchased a unit such as Savio Livingponds Waterfall filter, you can add extra media to meet your pond needs and possibly save money. For instance I have often substituted volcanic rock of different sizes (depending on coarseness of filtration needed) for bio and mechanical media in these and similar filter. This article about Aquarium filter media can also be applied to ponds: Aquarium (and pond) Filter Media)

Plastic grates and even rocks formed into basin can be used to construct the bio and mechanical filter for flow through either prior to a water all or after (prior is more common). If a cement product is used in construction, I recommend sealing it with a water proof sealer available at most Home Improvement Stores.
Use of volcanic rock for bio filtration and Coarse Polyester Media (pictured to the left) can help with the filtration need of your bio falls project.



*Another filter that works well with small and many medium sized ponds is the barrel style filter by Tetra and Coralife; these are excellent bio filters, but poor mechanical filters. They also are hard to hide as they have to use gravity to poor water back into the pond after the pump first supplies these filters with un-filtered water.
This filter too can be a DIY project using a large sterilized drum.

Pond Filters of Note & Larger Pond Filters:

*A filter that is also used is pool style sand filters. I have used these filters in many of my largest pond installations (over 5,000 gallons). Despite what pundits for these over priced monstrosities say, they are not originally designed for ponds. They tend to compact and the water then tends to follow the path of least resistance thru these filter, leaving areas on anaerobic bacteria producing poisonous Hydrogen Sulfide. DO NOT be fooled by their high price (often well over $1000), I can tell you from vast experience with them that they are not worth it!!
That said, if you already have one, I recommend adding volcanic rock and/or SeaChem Pond Matrix to improve upon this problem and also allow for nitrate removal via de-nitrification.





*Fluidized Sand or Moving Bed Filters.
These types of filters are purely for nitrifying (removal of ammonia & nitrites but not nitrates), however they are excellent at performing this task!
I do recommend pre-filtration of some sort such as coarse rocks near an intake or at the very least a coarse mesh screen to allow for maximum effectiveness.

Of these types, the Fluidized is the most efficient from what I have found, especially the third generation models such as the TMC V2 1500 Fluidized Filter. However these standard Fluidized Filters are only capable for ponds up to 500 gallons unless multiples are used or as a compliment to other filters.



The Nexus Moving Bed Filter or TMC Pro Pond Fluidized Filters should be considered for large ponds as their design lends itself better especially to ponds over 2500 gallons.

In fact at approximately $800 retail, you will not find a better aerobic bio filter than the TMC Pro Pond FBF80 or FBF160 Fluidized Filters with the capacity to handle 176 lbs and 352 lbs of bio mass respectively (this is a LOT of fish!). This works out on average to a 2500 gallon or 5000 gallon pond respectively. Their size alone belies their large pond capacity with the FBF160 measuring 61 inches tall and 13 inches wide.
For a serious pond keeper this would be the filter of choice by a wide margin, even over the Nexus Moving Bed Filter!

With either of these filters, I would strongly recommend a DIY Veggie/Bog filter to provide nitrate removal via plant roots end de-nitrification.

To the left is a diagram of a basic Gravity drain DIY in line Pond Filter (principle is similar to Tetra or Coralife barrel filters).

Click picture to enlarge

Volcanic rock makes an excellent filter media for DIY pond filters, waterfall filters, veggie filters (for the base), and many types of commercial filters such as the Savio.
Please click on the picture for more information.

For information about different filter media that can be used in your pond filter, please visit this article: Aquarium Answers: Aquarium and Pond Filter Media

[3] PLANT FILTRATION (Often called Veggie Filters);

This is a very important part of pond keeping and filtration. A well planted and diversified planted pond is VERY important for clarity (including Green Water), pond and fish health, and even fish breeding. Any plant with a good root structure that grows fast and has the majority of their leaves above water is a good candidate to start with. I recommend water iris for their strong root structure, fast growth, great nutrient absorption, and a great place for baby fish (fry) to hide feed and grow (if you use an “in pond” Veggie Filter).
There are many other excellent plants as well such as Sweet Flag, Parrots feather, Bluebells, Umbrella palm, Papyrus, & bull rush. Make sure these types of plants (plants with roots in water and leaves above) are planted in an area of good, but not strong water movement. This ensures that they will do their job as plant filters.

Other plants such as lilies and hyacinth are reasonable for nutrient removal too, but not at the rate of iris and similar plants. These plants do not have the root structure that will aid in the rapid removal of nitrates and ammonia. These other plants are useful for shade, which will slow algae growth.

I will off place this plant filter in an area of water constriction between two ponds or in an upper waterfall area with low to moderate current. In smaller Ponds I will just place this plant filter in a corner of the pond with a small water current applied to the planted area.
I prefer my plant filters within the pond as these look more attractive in my opinion and give the fish fry a place to hide (except in the case of the waterfall or cascade plant filter).
The area of the veggie filter should be relatively shallow as well to force water movement over and through the roots (the pictures above do NOT represent a proper veggie filter, just a picture of an umbrella palm and water iris). I also recommend ½” -2” rock under the plants to allow the roots to better establish themselves, do not use sand or soil.
Other methods include separate veggie filters in containers or pools. I am not as fond of this method for the reasons above, however these do work.

Click picture above to enlarge

FOR MY FULL VEGGIE FILTER ARTICLE, please visit this link:
“POND VEGGIE (PLANT FILTERS)”
This article includes much more information on this subject, as well as links to resources for bog plants and similar.
The bottom line is few ponds in my opinion are truly complete without a Veggie/Bog Plant Filter as they can make a big difference when done right in pond clarity (I had one pond I built for a client that had such a large and efficient Veggie Filter that it stayed clear without the UV Sterilizer running). For those who read this, please note that in my emphasis of the use of Veggie filters is not driven by financial gain, that although I obviously list products to sell through my many articles, I have little to gain since these are primarily a DIY products.

[4] UV STERILIZATION (& Algae Control):

Although not always necessary in a well planted, well shaded, well filter aquarium; they are still very useful. UVs help with algae control, disease prevention, and the Redox potential (which is important for fish health and proper filtration). For the UV to properly function in a pond, the flow rate should not exceed 20-45 gph per watt of UVC (lower flow rates of under 25 gph are best for sterilization while the higher limits of 45 gph are still OK for algae clumping/eradication). In larger ponds with high flow rates I recommend more than one UV sterilizer, with a by-pass from the main line, connected in parallel not in line together OR a separate, slower pump (with a pre-filter installed for the sole purpose to run the Pond UV sterilizer).
For this reason, I also do not recommend the popular filters (including the Clear Stream we sell as well as the Tetra or Cyprio pressurized filters) with built in UVs. The flow rate is usually too high to work properly , as well there is usually too much debris in suspension in these filters for the UVC radiation to properly work and the design of most I have seen and used have too high a gap between UV bulb/quartz sleeve sterilizer compartment wall, which is generally not adequate for good UVC exposure.
A separate UV is always best! I have maintained ponds with UVs in filters and UVs separate and the difference in water quality in the separate UV ponds is significant.

For more information about proper UV sterilization and how it works please visit this site; "WHY USE A UV STERILIZER"

Many times I have heard of complaints that their UV did not help with algae control, when I checked this clients pond, I found inadequate filtration and an improperly installed UV sterilizer. Even a properly installed UV Sterilizer cannot over come poor filtration and a poorly planted pond. And many manufacturers make claims of high flow rates that are impossible for proper contact time.


POND UV WITH BYPASS APPLICATION DIAGRAM; Click picture to enlarge.

To maintain a proper flow rate per watt per gph, connect a ‘T’ in line to your high flow rate pump, from there a reducing bushing (Ex. In 1-1/2” PVC I would step it down with a 1-1/2” by ¾ bushing), from there an in-line ball valve, from there to your Pond UV Sterilizer, then back to your pond or water feature (such as a waterfall or spitter).

Another option is to split the outflow from a large, high output pump and place a UV Sterilizer on each line.
As an example with a 3600 gph pump (of which these gph flow ratings are based 0 head pressure and after real world applications that include lifting of water the actual flow would be less) could be split and two TMC Pond Advantage UV Sterilizers (or equivalent)placed on each line You can also simply add a separate lower rate pump just for your Pond UV Sterilizer.

The picture to the left depicts a UV flow diverter for use with high flow pond (or aquarium) pumps so as to achieve correct gph/per watt flow rate through the UV Sterilizer.
Please click on the picture to visit page these are listed on or to purchase
These diverter valves are also listed/sold here: TMC UV Sterilizer/Clarifier; Extra Parts, accessories

Please note that these diverters only represent a fraction of PVC sizes and combinations, as well a ball valve may also be necessary on two sides of the diverter if head pressure is too high on the UV Sterilizer return side.

Here are a few solutions if the flow is too slow through your UV Sterilizer:
• Add a ball valve to the other outlet to force more water through the UV Sterilizer
• Lower the return for the UV Sterilizer to a place in the pond that is lower than the primary return, thus lowering head pressure through the UV and thus increasing flow.
• Do not recombine the two (or more) return lines down line before entering the pond, this interrupts water flow and will increase back pressure and reduce flow. All return lines must enter the pond separately!



This diagram shows a Via Aqua BH2000 AD Filter/Pump connected to a UV Sterilizer via the built in diverter valve. This valve is common to most similar fountains and filters such as the Pondmaster 1000, Lifeguard All in One and many more.
Please click on the picture to enlarge for a better view

Further Pond UV Sterilizer/Clarifier Suggestions;

For Small Ponds self contained Internal UV Sterilizers are growing in popularity, however I would caution prospective buyers that the first generation models (that are still sold by place like Amazon) have problems with electrical failure and are not efficient. The newer 3rd generation Internal UV Filter/Pumps (such as the SunSun 13 Watt; picture to the left) are much improved, however even these are only best for ponds up to 1500 gallons (and this is assuming other filtration). In most real world ponds I would not use this past a 700 gallon pond and again this is assuming other filtration (although multiple units can be used together for larger ponds).
I would also point out that while these UV/Filters do provide some filtration, this is intended primarily for pre-filtration prior to water entering the UV-C chamber. Do not believe web sites or other dealers stating this is an all in one filter UV. What the best of these is (such as the SunSun CUP-613 UV Pump/Filter); is a simple to use economical UV Sterilizer with filtration by itself maybe for a light bio load 100-200 gallon pond and otherwise additional filtration must be used (even a Hydro Pond Sponge Filter has vastly more bio capacity and would make an excellent inexpensive compliment to this UV Sterilizer.

For larger ponds (over 2000 gallons) often placing two UV Sterilizers such as two Terminator 36 Watt UV will be more economical than one larger UV Sterilizer as when used with separate pumps, the flow pattern is often better, therefore the pond "turnover rate" (how often the entire pond water passes through the UV Clarifier) is generally slightly higher than one unit of say 75 watts that often costs more than double the price as well.
The use of one pump with two UV Sterilizers (or more) can also work well, provided the returns are in different locations of the pond for optimum circulation and water turnover in the UV Sterilizer, however I have still achieved the best results when tow or more UVs are employed by using separate pumps with intakes and returns in separate locations of the pond to optimize water flow patterns through the UV Clarifier.

For very high volume pumps or extra large ponds units such as the TMC Professional 110 Watt UV Sterilizer would be an excellent choice. This is probably one of the best UV Sterilizers in its class, with high water exposure time even at high flow rates, without paying for often useless gimmicks. Although the advertised flow rate maximum is 9600 gph, I personally only recommend half this (about 4800 gph), however this still allows for the largest of water pumps (especially once one considers loss of gph when head pressure is applied by lifting through water features and the UV itself). As well this would allow this UV Clarifier to work via turnover of pond once every 2-3 hours of a pond up to 12,000 + gallons.

I should also note that spending more money for a pond UV Clarifier with wipers or HO UV-C bulbs is often not money well spent, as the wipers are often gimmicks that easily break and do little even when functional. As well HO (high output) UVC lights often are very short and even with this higher output, these are often still not enough for the water flow that is usually applied to these UVs. For example, although a 50 Watt HO bulb of 18 inches will handle a higher water flow of a 25 watt UVC bulb of 18 inches, the flow rate is not doubled.

Better is a unit with long exposure such as the TMC Professional 110 Watt UV Clarifier as compared a Smart HO Two-Lamp 100 Watt UV Sterilizer or two TMC Pond Advantage Premium 25 Watt UV Sterilizer as compared to one Emperor 50 Watt UV.
The TMC Pro 110 Watt is not only vastly less money, it is a superior UV Sterilizer in terms of UVC exposure and performance, as well the TMC Professional 25 watt is recognized by many pond keepers in Europe (where these are the #1 premium UV Sterilizer) as one best UVs for their ponds and two of these Sterilizers often cost less than many gimmicky 50 watt UVs!.

For an article that deals with the question of pond filter placement in relation to a UV Sterilizer (as well as many other aspects of UV Sterilizer us), please follow this link:
Is the UV best placed after or before a filter in a pond

Finally, while I recommend a UV Sterilizer/Clarifier for most ponds, I find that many pond keepers are too dependent upon this device where as if the UV ceases to function the pond immediately turns pea soup green. In my experience this indicates a pond that also is in need of better filtration, as the UV Clarifier should be a compliment to good filtration, not make up for poor filtration. The Veggie Filter I suggested earlier is a good idea or simply adding more filters if your pond has some filtration, but your current filtration is not adequate.

[5] POND ALGAE (Green Water and Blanket weed)

For MUCH more about the control, prevention and treatment of pond algae, please read this NEW article: Pond Algae; Green water and blanket weed

Also see this article for information specific to Cyanobacteria:
Cyanobacteria in Ponds