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A Pond Biological Filter

How to decide what biological filtration will work best in your pond.

By Stephen M. Meyer

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Q. We plan to install a biological filter for our 1,500-gallon pond. We thought this would be a fairly straightforward process until we discovered that there are so many different types of media to choose from. Gravel, lava rock, foam pads and bioballs are just a few. There are also all kinds of materials that look like mattress padding, carpet padding, wall insulation and so on.

Each product claims to offer more surface area than the other. One dealer says that the medium he uses has so much capacity (about 200 square feet of surface per cubic foot of media) that we will only need a small 30-gallon filter to treat our entire pond!

Some of these materials are incredibly expensive. In fact, if we opted for what is said to be the best material, we would have to put off buying any fish for the pond for at least one year. Can you help?

A. Until a few years ago the primary debate about filtration media among pond filter experts was limited to the question of which size gravel was best: pea gravel, 3/4-inch or 1-inch. Gravel was basically all there was. When I began to push the idea of using lava rock, which I found far easier to keep from clogging, there was much criticism! It was considered unthinkable to use anything except gravel.

Today, the variety of materials available for use as pond filter media seems to grow monthly. And, while I am a great fan of diversity, it seems to me that some of the claims are getting out of hand. In particular, if the filter material you are considering is so expensive that it prevents you from putting fish in your pond, something is wrong. The purpose of the pond is to enjoy fish outdoors. The purpose of the filter is merely to make a better environment for the fish.

The unfortunate truth is that many of the claims made about the new "supermedia" for pond filters (and aquarium filters) are just hype. And like the price tags they carry, they bear no relation to reality.

As your letter notes, the capacity of a biological filter is strongly related to the surface area it provides for nitrifying bacteria to colonize. All else being equal, if one medium provides 10 square feet of surface per cubic foot of media and another can provide 20 square feet per cubic foot, then the latter will allow twice as many nitrifiers to colonize. It will offer twice the capacity of the former.

To a great extent this method of analyzing the problem works well for solid media, such as gravel, and for high flow-rate media, such as bioballs and hair curlers. And, because different types of media surfaces may have different degrees of attractiveness to the bacteria, there are limits, even here — more on this later.

When it comes to semi-permeable and permeable media, such as those that are in vogue now, comparisons of alleged surface area per unit of volume are worthless. This is because the estimation of so-called "internal" surface area available for bacterial colonization is a purely mathematical calculation based on the physical structure of the material, not its operating characteristics.

For example, a foam block that claims 200 square feet of surface area per cubic foot of material may actually offer something close to this when it is just out of the packing crate. But once it has been in a pond filter for a few weeks and its internal spaces get filled with algae, organic and inorganic material, and dead bacteria, its effective surface area will revert to its outer surface and probably be no more than 8 to 10 square feet of surface area per cubic foot of material. The most intense cleaning will never return it to the mythical state it arrived in.

Solid media, such as gravel, are not as greatly affected by the operating environment because their surface area calculations are based only on outer surfaces and are not artificially inflated by highly deceptive internal surface estimates. Of course, even gravel beds clog and, as a result, the effective surface of a gravel filter will decline over time, by perhaps 50 percent. But this is minor compared to the 95 percent decline for the supermedia.

The surface area to volume figures for high flow-rate media, which do not clog under normal conditions, present a fairly accurate estimate of their effective operating surface area. Such media, however, can be fairly expensive in quantity.

You should not be fooled by the argument that nitrifying bacteria prefer coarse surfaces to smooth ones. This argument has been used to call into question the value of plastic media such as bioballs. As it turns out, you cannot predict the attractiveness of a media surface based on its relative roughness. Plastic, for example, beats out cotton and several other materials by a mile in bacterial count testing.

My recommendation is to design your biofilter so that it is about 10 percent of your pond volume, irrespective of the medium used. This would be about 150 gallons (20 cubic feet) in your case. Any medium — gravel, lava rock, bioballs, stacked foam blocks — will do a fine job in this case. The only drawback to gravel is that it is a pain to unclog, but even here pondkeepers have designed air blowers to ease the job.

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