How Many Fish Is Too Many For My Tank?

If you’re new to owning fish, you may be wondering how many is too many for your tank. The answer isn’t simple. Although some calculations are based on the tank’s volume, that isn’t the most accurate way to determine the maximum number of fish to have. The most important factors in figuring out the right number are surface area and fish type.

Surface Area

Relying on volume is inaccurate because it doesn’t account for the tank’s shape. Surface area, however, does. Oxygen diffuses into the water through the surface, so the more surface area there is, the more oxygen there will be, too. This means that a wide tank will have higher oxygen levels than a narrow one of the same volume. When there is more oxygen available, more fish can be supported. You can use special calculators online to find out the surface area of your tank.

Fish Type

The fish you have are even more influential in defining how many is too many. Fish that are slender, such as tetras, use less oxygen than species with fuller bodies, such as goldfish. Cold-water and marine species need more space. If you get young fish, remember they will need more room and oxygen as they grow. Fish that are aggressive may do better with fewer companions, whereas schooling varieties need many. Add fish to your aquarium slowly to allow adjustments, and watch for problems.

Other Things to Consider

You must properly maintain your aquarium in order for it to sustain any aquatic life. Also, remember to exclude plants and small invertebrate in the population count. Finally, it’s best to have a Big Al’s air stone or power filter, too. These increase oxygen supply by disturbing the surface to allow for more gas exchange.


With so many factors involved, it can be hard to establish the perfect number of fish to house together. These guidelines give you a great starting point. When in doubt, settle with fewer fish per tank.

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