What is Biological Filtration?

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A biological filter is one of the key pieces of equipment in an ecosystem pond. Biological filtration is a general term referring to the process of using beneficial bacteria to clean the water on a molecular level. In a natural stream or river, bacteria colonize on the stones and pebbles at the bottom of the stream, which naturally breaks down toxins as the water flows around. It is this natural process that manufactured biological filters attempt to emulate. However, in addition to cultivating bacteria at the bottom of the pond like in nature, we will also need to colonize bacteria in a biological filter to give the pond some extra help. The pond water is forced to flow through the filter via the pumping system, and as it does so, the beneficial bacteria can work to break down toxins in the water.

Why You Need Biological Filtration

One of the main uses for biological filtration is to convert harmful ammonia, which is toxic to fish and bad for a pond ecosystem, into nitrates. Ammonia is naturally produced from fish waste and other decaying matter in a pond. This is why a pond skimmer is so important—a skimmer will trap floating debris before it has a chance to sink to the bottom and decay. After ammonia is converted into nitrates, we have a much more manageable situation on our hands. Nitrates are harmless to fish in reasonable quantities and are actually a plant nutrient, acting as a source of food for plants in the pond.


However, any time you introduce plant nutrients into a pond, it’s important to keep in mind that certain types of pesky algae like blanketweed can also benefit. Therefore, it's a good idea to try to manage the levels of nitrates in your pond. There are a couple ways of doing this. The easiest way is simply to plant aquatic plants in and around your pond that will naturally feed on the nitrates, leaving less nutrients to support algae growth. Another way is through scheduled partial water changes, which replace a portion of the pond water with new nitrate- free water. Since we’re building an self-contained ecosystem pond, we’ll focus more on using plants to eat up excess nitrates than on relying on partial water changes, as might be necessary in a dedicated koi pond.

Choosing a Biological Filter

For our ecosystem pond, to keep things simple we’re going to stick with one of two types of filters:

1) Waterfall Filter

Most ecosystem ponds will include a waterfall or water feature of some kind. (See more in 1.6 - Waterfall Design.) In our opinion, the best way to incorporate biological filtration into a pond with a waterfall is to use a biological waterfall filter. Not only do waterfall filters provide the biological filtration we need, but they also act as an excellent base for building a great looking waterfall with less effort. The filter acts as both a water collection pool at the top of the waterfall and a waterfall spillway to provide an even cascade of water. It’s important to note that a waterfall filter requires a bit of care to properly conceal into the landscape. There are a few tips you’ll want to follow when choosing and installing the rocks to properly conceal the filter. We’ll cover the specifics of this in the building section later.


A biological waterfall filter before being disguised with rocks

2) External Pressurized Biological Filter

If you’re not planning to build a waterfall at all, or you want a waterfall that does not meet the design requirements for using a waterfall filter (such as in the case of a formal water garden), then you can install a stand-alone biological filter that sits outside of the pond. The filter has an inlet and an outlet. How you channel the water from the outlet is your choice. For a more tranquil pond, you could simply return the water in a low-key way. You could also hook it up to a fountain or create a clean waterfall design with a piece of equipment like the Atlantic Colorfalls (pictured below).

Aqua Ultraviolet Filter


Atlantic Colorfalls

There are a number of different external biological filters on the market. For an ecosystem- based pond, we’d recommend sticking to a pressurized filter. Pressurized means that the entire system is under the pressure of the pump and has the capability to return water to the pond, regardless of the elevation of the individual pieces. For example, if your filter was sunk into the ground and was actually below the elevation of the pond surface, the filter would still be able to send water back into the pond. The alternative to a pressurized filter is a gravity filter, which relies on gravity to move water from place to place. Gravity-fed systems are popular in dedicated koi ponds.

Sizing a Filter

Much like with skimmers, it’s important to get the right size of filter for your pond. Every filter on the market will include either a recommended flow rate, or a minimum and maximum flow rate that the filter can handle. It’s very important to pay attention to these numbers when choosing a filter so that your filter works as intended. For example, a filter might accept a minimum flow rate of 1500 gph and a maximum flow rate of 3000 gph. That means that you’ll want to make sure your pumping system is moving water somewhere in that range—say, 2000 to 2500 gph. Later, in the section on plumbing, we’ll go into greater depth on determining a proper flow rate for your pond.

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