How Many Fish Can I Put in My Aquarium?

Weclome to How Many Fish. You can do three things on this page:

1. Calculate the surface area of your tank
2. Calculate how many fish your tank can support
3. Learn what the results mean

Results available in U.S. Units and Metric.
Have a Hexagon Aquarium? Our hexagon tank page has calculators for size in gallons or liters and surface area in inches or centimeters.

What the Results Mean

This calculator uses the "surface area" method. The surface of your aquarium is where oxygen diffuses into the water. The more surface area, the more easily oxygen diffuses, and therefore the more fish your aquarium will support. For example, a tall, narrow aquarium of 29 gallons cannot support as many fish as a standard rectangular 29-gallon tank. NOTE: Calculations assume the use of an airstone or a filter that disturbs the surface. Adding more airstones will not increase fish capacity.

This only a rule of thumb. The body type of the fish also makes a difference. Slender fish such as tetras use less oxygen than full-bodied fish such as goldfish. The calculator reflects this by providing two extremes. For slender fish, we use the value of 12 square inches of surface per length-inch of fish. For full-bodied fish, we use the value of 20 square inches per inch of fish. Cold water tanks and marine tanks have higher surface requirements. Cold water fish need a minimum of 30 inches of surface area per inch of fish; marine fish need a minimum of 48 inches of surface area per inch of fish.

The results apply to the full adult size of the fish. Most fish when purchased are juvenile and much smaller than their full-grown size. For example, a Red Tail Shark may be two inches in the pet store, but may become five inches within a year! Be certain to research how big your fish will grow, and use that full-grown value to calculate how many inches of fish your tank will support.

The results assume a properly maintained aquarium. This means regular partial water changes, adequate filtration, monitored water chemistry, etc. If these conditions can not be met, then the proper number of fish in the aquarium is zero.

Temperament and other needs trump this rule of thumb. For example, aggressive fish may need more space for their size. Schooling fish will need a minimum number to exhibit shoaling behavior or even thrive. Some species, such as oscars (a popular pet store fish), need to be alone (and will grow rather large). Select species carefully after determining your general capacity.

Snails and shrimp do not count. In normal populations, thier bio-load is small, and they serve to recycle fish byproducts. Likewise, plants can be ignored.

Add fish slowly! Your tank may support this many ultimately, but don't add them all at once! Add a few fish at a time, a week or more a part, to give the aquarium time to adjust. Too many fish, even the "right" fish, can overload the natural bacteria that need time to become established. Read more about this on our cycling page.

Do you know about cycling a new aquarium? Yes, we are repeating ourselves, but this is very important. The biggest mistake made by people setting up their first aquarium is to not understand the concept of cycling. Learn more here on our Cycling page.

How about a hexagon aquarium ? We have a separate page for hexagon aquariums. Visit our How Big is My Hexagonal Aquarium? page to calculate the size in gallons or liters, and to determine how many fish are appropriate for your size of hex aquarium.

Frequently Asked Questions

Q. What about the one-inch-per-gallon rule?
For starters, the one inch per gallon rule only works for tropical freshwater fish. Also, it does not account for the shape of the tank, how full it is filled, or how much water is displaced by gravel and decorations, nor does it account for the shape of the fish. However, for standard-shaped tropical freshwater aquariums, the one-inch-per-gallon rule gives a similar result as the surface area rule. Regardless, the adult size and body type still needs to be considered.

Q. Why won't more airstones increase the number of fish supported?
An airstone does improve gas exchange by keeping the surface of the water stirred. Many power filters do the same thing. You should have at least one or the other. However, the advantage is relatively small, and adding more will not make a significant difference. This surface area calculator assumes the water is being aerated in some way.

Q. I posted the results of this calculation on a forum, and people disagreed. What gives?
Experienced fish keepers know that finding the right balance of fish for the size of the aquarium depends on many factors, such as aquarium shape, aquarium decoration, fish size, fish shape, fish behavior, water quality and the maintenance routines of the owner. They know that any rule-of-thumb has its limitations. Just remember this calculator is only giving you a guideline to get you started; you will need to do further research into the species you want before you purchase.

Q. Is there a more precise way to perform this calculation?
The real equation will incorporate the gas-exchange rate of the tank and the biomass of the fish. To get the biomass, you would need the adult weight of the fish. Good luck finding useful data on that. Just start with the above calculator, then research the needs of the individual species you want. If in doubt, understock.