Many times when starting a tank, the most popular and very important question is: How many fish can I keep in my aquarium? Of course there are many problems with over-stocking an aquarium such as insufficient oxygen, lack of space etc. Most of our exotic fish will take a lot of punishment before coming to the surface to whisper their complaints by breathing atmospheric air. They can be overcrowded without any such demonstration.
Many times the aquarium is in beautiful condition but the fish do not grow. This brings out the fact that the fish have neither enough "elbow room" nor oxygen and sometimes underfed. The rule of thumb I have heard many times is "1 inch of fish to 1 Gallon of water". This, however, is only a rule of thumb, and many times it is hard to apply it to a specific fish.
For example, let's compare a 3" neon tetra, 3" discus, 3" tiger barb, 3" betta, and 3" cichlid. Each fish has different requirements. A 3" discus would not be able to live in a tank suitable for a 3" betta because of space. Even a 3" neon and a 3" betta have different requirements because the betta is a labyrinth fish. And if we take it even further, a 3" barb and a 3" neon need slightly different requirements as barbs are taller in body and neons slimmer. I could name many more examples but I think that you can see my point. Each fish has its own requirements.
The following then can be used to determine the number of fish in an aquarium. The first step is determining the surface area of the tank. Keep in mind when using the size of fish; it has to be the maximum growth the fish will reach.
For fish the size of grown Guppies, 3 square inches of air surface per fish. That is to say, an aquarium 9 x 20 inches, with an air surface of 180 square inches, can safely support 50 grown guppies, giving each over 3 inches. Other exotics the same size rate a bit more, as Guppies have great powers of standing overcrowding.
Labyrinth fish (Bettas, Paradise, Gouramies) need about half the amount of air surface calculated per fish of the same size.
Swordtails, large Platies, etc., need about 8 square inches per fish (4 x 2 inches, or equivalent).
Medium Barbs, 3.5 inches in size, and other fish of equal weight should have 20 square inches per fish (4 x 5 inches, or equivalent).
Large Barbs and Cichlids of 5-inch length require a minimum of 54 inches per fish (6 x 9 inches, or equivalent).
These are minimum requirements, not taking into account plants or aeration and at a temperature of about 74 degrees. For health, growth and first class conditions, the air-surface per fish should be doubled or even tripled. A large aquarium can support a little higher percentage of fish in proportion to its size than a small one.
By: Ron Reisdorf