Pond Filtration Systems

Fish and other wildlife produce biological wastes and hence, adequate pond filtration systems are extremely important in a fish pond.Pond filter systems also help in aerating the water effectively helping your pond fish get much needed oxygen supply.

Filter sizes and designs are very important, the media can be rock, preferably 2" smooth rock, or bio-filter media that consists of fiber mat, cell foam, brushes, etc., and plastic media, which can be expensive but less likely to clog or channel, is lightweight and easy to clean.

Sizing the filter to your pond depends on the size of your pond and how much sun, or shade it receives, plant types and numbers and the wildlife or fish that you keep. Keep in mind that clear water can still be contaminated by colorless toxins such as ammonia and nitrates.

As we have mentioned in other parts of our site, pond filter systems should filter one half of the ponds' volume in one hour. For example, a 500 gallon capacity would require a filter capable of moving 250 gallons of water per hour. Also consider that plumbing sizes are very important, for if your piping is too narrow the flow will be compromised and your pump and filters will work harder to clean the water.

Basically, there are two different kinds of pond filtration systems: biological and mechanical.

Biological filters

Biological filters turn nitrites into nitrates, bacteria that requires oxygen to survive. In using this type of pond filter system, the pump must run 24-7 for if it stops, bacteria use up the oxygen and die. In order for this system to work there needs to be a lot of bacteria, that anchor themselves to things such as plants, requiring a lot of surface area for the bacteria to grow. This means better filtration by biological pond filtration systems. Garden pond plants, by the way make great biological filters.

Biofilters need to be sized according to ammonia load, or the amount of fish waste produced each day. Selecting your pond filtration systems will depend on how many fish you plan to purchase and the weight of the food fed daily. Koi fish eat about 1-3% of their weight daily, and they consume the natural foods found in a pond. It will not be easy selecting a filter to match the load.

Bio filter media comes in many forms, from bio balls that have ridges that trap the bacteria in a surface area form but are very pricey, to lava rock that does the same job at a far better price. The problem with lava rock is that it clogs in a season and then needs to be cleaned. In between these are ribbons requiring large quantities to perform, foam that clogs quickly and ceramic, another pricey option.

A "down home" bio filter, easily obtained and inexpensive too, is nothing more than floor scrubber pads. According to some who have gone the trial and error route with their ponds, this media, when packed loosely in the filter, will not only do the job but do it so well that you do not need to clean the media. By just adding a few water hyacinths or taro plants, to name a few, to the top of the media, in no time you will have a great filtering system. In the spring, add beneficial bacteria to the filter and do not cover it because the sun helps the whole process along.

Mechanical filters work well with biological filters in keeping the water clean by skimming solids through the "swirl chamber", thus allowing the solids to settle. As the water flows up through this media, it is oxygenated for the return to the pond. If your pond has a waterfall or stream, this process happens naturally.

Mechanical filters

Mechanical pond filtration systems will capture most of the large waste, particles and sediment but do need to be frequently cleaned. There are several different types of mechanical filters in the marketplace: skimmer filters mentioned earlier, with the box skimmer more easily maintained over the floating type. There are vertical and horizontal filter mats. Vertical mats require more maintenance and need to be cleaned more often. Horizontal mats can be cleaned once a month, are made more sturdy than the vertical mat, filters more debris, never clogs so that there is no danger of the pump running dry.

External filters

External pond filtration systems are usually larger than other pond filters and are much more easily cleaned due to accessibility to the inside filter housings. The filter incorporates the benefits of biofiltration with larger foam filters and even "fish doors" to prevent fish from entering on skimmer filter systems and becoming trapped. Situated close to the pond, the filter is easily disguised by plants and rock, or buried up to the filter top next to the pond, but still very easily opened for cleaning and winterizing.

These filters can go long periods without cleaning and due to their size, are perfect for larger pond and fish loads. Some external filters offer filter fall units, an additional filtering advantage. Be sure to look for filters that can continue filtering even if the media clogs and contain room for several different filter media.

The one drawback that we can see with the external filter is the absolute need to plumb properly with the right size tubing and fittings. For the beginner, this could mean consultation with an expert to get it right. All filters run side by side with the pump to do the job of keeping your pond clean and safe.


Other pond filtration systems that should not be used are pump pre-filters, also requiring frequent cleaning. Submersible filters can be good but because they remove ammonia, a by-product of fish waste, they need to be cleaned often to maintain flow. Above all, do avoid swimming pool filters because sand and diatematius earth filters will harden in a short time because these media are not designed to filter fish waste.

Filters generally process 0.1 gram of food daily per square foot of filter media area. Surface area of the pond filter system is very important because the larger the area, the more biological activity it will support. Measurement of surface area of the media is per cubic foot. As we have mentioned before, don't overfeed your fish because this will cause the ammonia and nitrite levels to rise dangerously high, unless you are prepared to change the pond water frequently. Over feeding puts too much stress on a biological filter, especially when you consider the square footage of your pond. You may have to set more than one filter in a series depending on the total food being processed.

Ultraviolet filter

Last but no means least is the ultraviolet filter that controls green water algae. The drawback with these type of pond filtration systems is that they destroy beneficial bacteria that comes in contact with the tube housing the light bulb. The UV filter works best when used with a mechanical or biological filtration system. If you are looking to treat your pond naturally, the UV filter is not a good idea, because of the radiation given off. These filters are very expensive not to mention the bulbs that need to be replaced every six months and are also expensive.

In the world of pond filtration systems purchasing the best filter that your budget will afford from reputable manufacturers, is probably one of best pieces of advice that most pond keepers will give. If your pond is suffering, your fish will suffer too. For additional help, click on our pond calculator page and go to the pond planner, input your desired turnover rate into the calculator to find the gallons per hour filter size required.

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