Fish as Filters

As most of you know, one of the most frustrating things about aquarium keeping is the need to filter noxious compounds from the water. These compounds are the metabolic products of living things in the tank. The most troublesome of these is oxygen. While our plants need oxygen for respiration, they produce far more oxygen during the day than they need at night. The result is that oxygen accumulates until the water is saturated with it, then it starts to bubble to the surface.

This oxygen saturation has two major effects. First, it oxidizes trace elements to a form that is not readily absorbed by the plants. We spend a considerable amount of money in trace element fertilizers to ensure that our plants are at their best health. The last thing we want is for these trace elements to be oxidized before our plants can absorb them. Second, oxygen promotes the growth of nitrifying bacteria. These bacteria convert ammonium to nitrite and then nitrate. Our plants would much rather deal with ammonium than nitrate, and Cryptocoryne species will rot after absorbing nitrates. Again the efficiency of fertilizers, in this case ammonium fertilizers, is drastically reduced.

Needless to say, this excess oxygen must be removed if we want to keep healthy plants. One low-tech solution is to introduce an airstone into the tank. This is inexpensive though the hum of the air pump and the resulting bubbles may not be a welcome addition to our serene aquarium setting. Furthermore, the bubbling will try to equalize the concentration of other gases in the water to that of the atmosphere. If we inject carbon dioxide in the water, the bubbling will reduce the levels of carbon dioxide. Also, there is no way to control the concentration of dissolved gases. A much more effective solution is the use of one of the deoxygenation units that are now available. The German ones are very precise and allow us to set the gas concentration to a desired level so there is enough oxygen for respiration and the carbon dioxide remains in solution. Unfortunately these units are hideously expensive and difficult to find in North America.

But while doing some research I stumbled upon what is a low-tech and efficient solution to the oxygen problem: animals, particularly fish. Animals require oxygen for respiration like our plants, but unlike them they are not photosynthetic. They cannot generate oxygen. Furthermore besides carbon dioxide their metabolism creates nitrogen compounds, particularly ammonium, and phosphates. Fish in particular are very suited to our purpose as they come in various shapes, sizes, and colors to suit our aquascaping. Further research revealed that the cost of the food required to keep fish healthy is much less than the cost of ammonium fertilizers. What we have here is a cheap, low maintenance deoxygenator and fertilizer dispenser, all wrapped in a conveniently shaped package.

Fish have other advantages too. Some are algae eaters and will be more than pleased to graze on any algae that finds its way to our tanks. This is particularly helpful in the early stages, just after the tank has been set up. Other fish are nocturnal, so they spend their days hidden out of sight. These fish do not need to match our decor as they will be often unseen. Also for us who use sand as a substrate, and are tired of raking the sand with the Amano aquascaping rake, there are fish that feed off the substrate. These fish, particularly “loaches” and “corys”, are always digging in the sand in search of food, ensuring that it does not form a hard crust. Loaches are also known to eat snails, which could be either a good thing or a bad thing depending on the type of snails one has.

Some people discover that they enjoy fish so much that they seem to forget the original purpose of the aquarium and begin to ignore plants. Some even go to extremes and set up “fish-rooms” full of fish-only tanks. Others, perhaps in an attempt to disguise their dislike for plants, buy elaborate and expensive plastic plants. I will not discuss these extremes here, but I would like to mention that fish are not without their disadvantages. Some fish, particularly cichlids, are known to destroy plants and rearrange the substrate as part of their territorial behavior. Others are notorious plant eaters and some, like the guppy, breed like duckweed and threaten to overpopulate our tanks. Therefore one must be careful about the fish selection to ensure the fish are compatible with our plants and each other. Unlike plants where size differences aren’t a problem, larger fish tend to eat smaller fish. Luckily now there are many books that describe fish and their habits in detail.

What we don’t have though are tables listing the efficiency of fish as oxygen consumers, by size and numbers, in relation to temperature. One must start with a few fish and gauge their ammonium production through test kits or algae growth. Though not easy to measure, oxygen levels should decrease after fish are added. One beneficial side effect of fish food is that it can also be consumed by protozoans and anaerobic bacteria with the beneficial results ammonium production. So there is no need to increase the fish population if the ammonium levels are still too low, it is just a matter of feeding more. Too much feeding is not a good idea though, as the bacteria will cloud the water.

So, even though we cannot expect to have a perfect balance between plants and fish in our aquaria, the two are complementary and contribute to a more balanced setting. For those of us on a low budget or who prefer a more balanced approach to our hobby, fish offer a good solution to the oxygen and fertilizer problem.


Riehl, Rüdiger, & Baensch, Hans A. (1986). Aquarium Atlas. Melle, Germany: MERGUS-Verlag.

Scheurmann, Ines. (1985). The New Aquarium Handbook. Woodbury, NY: Barron’s Educational Series Inc.

Verde, Polegar. (1992). Besteiras Sobre Plantas e Peixes no Aquário. Rio de Janeiro, Brazil: Editora Primeira de Abril.?