Besides mechanical and chemical filtration, another method is known as “biological filtration.” It works the same way in a pond as it does in nature. Placing a filter medium inside a box creates a living “Petri dish” where beneficial bacteria can grow, just as they do in the wild. (Although in the wild, the “Petri dish” is usually peat moss or plant roots or anything else to which beneficial bacteria can cling.)

As bacteria grow, they consume waste byproducts. That makes a biological filter the single best type of filter for fishponds, because beneficial bacteria actually consume toxins that are harmful to fish.

Fish, like all animals, generate waste in the form of ammonia (NH3). If too much ammonia builds up in your pond, your fish’s gills will become irritated. Long-term exposure to ammonia will eventually harm a fish’s internal organs. In nature, the aerobic bacteria, Nitrosonomas, removes the hydrogen and adds oxygen to each ammonia molecule, thus converting it chemically to nitrite, or NO2.

Nitrite, a highly toxic chemical, is a by-product of the naturally occurring Nitrogen Cycle. The bacteria, Nitrobacter, converts nitrite into nitrate (NO3), which is much less toxic to fish.

Biological filtration works by attracting and holding both Nitrosonomas and Nitrobacter bacteria. When exposed to oxygenated water (the kind you would find in a pond!), these two bacteria begin converting ammonia to nitrites and nitrites to nitrates. To be effective, a biofilter must have a large surface area on which the bacteria can form. This surface area is known as “biomedia,” literally a medium on which beneficial bacteria live.

Many materials have been used as biomedia. These include rough plastic hollow tubes, orange bags, lava rock, foam sheets, and even hair curlers. The problem is that most of these materials take up a great deal of space, meaning that your filter box is going to have to be roughly the size of the family car in order to generate enough bacteria to do the job at hand.

In our biofilters, we use a combination of foam sheeting and soda straws. Yes, the kind the kids drink from. We bundle them up and weight them down (so they don’t float away) and place them inside the filter box. Each straw, because of its length and shape, offers a large surface area upon which the bacteria can congregate. When you multiply the surface area of a single straw by a hundred all tied together, you begin to get an idea of just how much beneficial bacteria your biofilter could generate! Best of all, drinking straws are available everywhere dirt cheap!

Once a biofilter converts ammonia into nitrites and nitrites into nitrates, the nitrates (which are only mildly toxic to fish) can be removed through regular water changes. They can also be removed by water plants, which absorb the chemical through their roots, where they give off oxygen in return. Since the number of plants required to accomplish the task are far beyond the scope of most pond owners, however, frequent water changes are still considered the best way to remove excessive nitrates.

There is a drawback to biological filtration. Although it’s a minor one, it could prove deadly for new pond owners. It takes from several weeks to several months for the bacteria in a biofilter to build up to a level where it can complete the nitrogen cycle and begin protecting your fish. Until then, although your new pond will look clean and healthy, there will come a time when the ammonia, nitrite, and nitrate levels will spike way up.

In order to prevent a sudden die-off of your prized fish before the nitrogen cycle takes hold, you’re going to have to make frequent partial water changes, especially for the first few months of your new setup.

Check out D. J. Herda's two latest gardening books, Zen & the Art of Pond Building and From Container to Kitchen: Growing Fruits and Vegetables in Pots, both available from Amazon.com.

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