How many fish can you keep in an aquarium

There are several things to consider in deciding how many fish you can keep in an aquarium or fish bowl: surface area, water movement, filters, fish conflicts, and how experienced you are.

Surface area, water movement, oxygen and carbon dioxide

To calculate the surface area measure the length and the width of the area where the air touches the water, and multiply to get the surface area. If the sides of your aquarium are straight up and down just measure the length and width of the top of the aquarium. If you have a barrel type bowl you measure the length and width at the water line.

If the surface area is round measure the length across divide by two, to get the radius, square the radius, and multiply by pi, 3.14. And you thought you would not use the math you took in school.

In many bowls and jars you can frequently increase the surface area by lowering the water level. This gives the fish less room to swim, but increases gas exchange. This is a good idea if the fish are crowded. Room to swim is not as important as oxygen, and getting rid of excess carbon dioxide.

Sometimes you can increase the surface area by rasing the water level, if you do not have a cover make sure they can not jump out. For small fish I try to leave two inches.

Water movement greatly increases the exhange of gases. Both air stones, and filters keep the water moving. Many times this is done by pumping air down into the aquarium and letting it bubble up to the surface. The bubbles do not force much oxygen into the water, it is the water movement at the surface that facilitates gas exchange. So a water pump is just about as effective in increasing oxygen content, and getting rid of carbon dioxide as an air pump, assuming that both move the same amount of water.

A small fish the size of an adult guppy, neon tetra, or zebra danio, say about an inch and a half in length, should have at least three square inches, 19 square centimeters, a fish, if there is no water movement. You can double the number of fish if there is water movement.

A typical 10 gallon tank is ten inches, 25 centimeters, by 20 inches, 50 centimenters. This is a total of 200 square inches, 1250 square centimeters. It can hold 66 full grown guppies or neon tetras without water movement, or twice that, 132, with water movement. My source for this is the Innes book, Exotic Aquarium Fishes, which for many years, perhaps decades,was the Bible of tropical fish keeping. Innes was the dean of the hobby, the scientific name of the Neon is named after him. I have followed the advice of this book for a long time, so I am also speaking on the basis of experience. These are maximum numbers, both Innes and I suggest that you double or triple the amount of surface area per fish for growth and better health. Furthermore, if you are using a filter, airstone, or other method to keep the water moving I would suggest that you avoid having more fish than can survive without water movement. That way if the electricity goes out, or a pump breaks the fish will still survive. There is a big exception where you may want to have the maximum number of fish. This is when the fish are breeding and you are trying to raise a large number of fry. Even in this case, however, you might want to get rid of the excess fish so the others will grow faster.

Other Pages On Bowls On This Site

Home Page on Fish Bowls
Which Fish Thrive in Bowls
How To Clean A Bowl
Biological Filtration Without Electricity
Italian Towns Outlaw the Fish Bowl