Aquarium Estimator Tools

"How Many Fish?" Aquarium Stock Level Calculator

Estimate how many fish an aquarium can support

This calculator uses surface area to estimate how many fish can be safely stocked in a rectangular aquarium.

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Do you want to know "How many fish can I put in an X gallon tank?" This method for estimating how many fish a tank can support is superior to the traditional a "how many fish per gallon" or "per litre" rule of thumb, such as "one inch of fish per gallon" or "one centimeter of fish per litre". Please read the What Results Mean and the FAQ sections following the calculator tab to learn more. To get results for hexagonal aquariums, see our hexagonal tank tools.

Fish Stock Calculator: U.S. Units

(Scroll down for Metric Units)
Enter your values. Click "Solve" for results.

Tank Length (across front) inches
Tank Width (front to back) inches
Fish Stock Level Results
Calculated Surface Area in2
Maximum inches of slender fish your
tropical freshwater tank can support:
Maximum inches of full-bodied fish your
tropical freshwater tank can support:
Maximum inches of fish your
cold freshwater tank can support:
Maximum inches of fish your
salt water tank can support:

Fish Stock Calculator: Metric Units

Enter your values. Click "Solve" for results.

Tank Length (across front) cm
Tank Width (front to back) cm
Fish Stock Level Results
Calculated Surface Area (sq cm)
Maximum cm of slender fish your
tropical freshwater tank can support:
Maximum cm of full-bodied fish your
tropical freshwater tank can support:
Maximum cm of fish your
cold freshwater tank can support:
Maximum cm of fish your
salt water tank can support:

What Your Stocking Level Results Mean

This calculator uses "surface area" to determine fish stock levels rather than gallons or litres. This is better than a calculator that uses volume alone, such as the common how many fish per gallon rule. (Reference: The Complete Aquarium by Peter W. Scott.) 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 29 gallon aquarium cannot maintain as many fish as a standard rectangular 29 gallon tank because less water surface contacts air. NOTE: Calculations assume the use of an air stone or a filter that disturbs the surface. Adding more air stones will not increase the number of fish your tank will support.

Not all fish are alike in their oxygen needs. Slender fish such as tetras or danios use less oxygen than full-bodied fish such as goldfish or oscars. 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 (or 30 sq cm/1 cm of fish) For full-bodied fish, we use the value of 20 square inches per inch (50 sq cm/1 cm) 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 (76 sq cm/1cm); marine fish need a minimum of 48 inches of surface area per inch of fish (122 sq cm/1cm).

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

Fish species characteristics and individual temperaments supersede this stocking guideline. Fish that are particularly large, long or active will need dimensions than allow them the physical space they need to be healthy. For example, oscars, a popular pet store fish, produce a a great deal of waste and so need more volume than this surface-area might suggest. Aggressive fish may need more space for their size (for their tank-mates to stay out of their way.) Schooling fish will need a minimum number to exhibit shoaling behavior or even thrive. This capacity calculation is only a beginning; research your choices for all the factors that should be considered.

Snails, shrimp and plants do not count. In normal populations or typical small sizes, invertebrate bio-load is small, and both invertebrates and plants serve to recycle fish byproducts. Of course, large specimens in a marine tank, for example, should be counted as fish.

Add new stock slowly! Your tank may support this many ultimately, but don't stock them all at once! Stock a few fish at a time, a week or more a part, to give the aquarium residents time to adjust and the biological waste processing bacteria time to expand to meet the new demand. 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.

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.

Example Maximum Tropical Aquarium Stock Amounts

The following table shows stocking a single species of fish to maximum capacity. This is only for illustration--some species don't even like to be in groups of their own kind! Furthermore, the stocking levels shown do not account for the characteristics or needs of a particular species. In particular, larger fish (e.g., oscar) need a tank large enough for them to move freely; meeting surface area needs may not meet physical size needs. Research your candidate species. If in doubt, under-stock.

Example Maximum Stock Amounts if only one species in tankBody TypeMax Length CMMaximum Length InchesHow many fish in a 5 gallon tankHow many fish in a 10 gallon tankHow many fish in a 20 gallon tankHow many fish in a 50 gallon tank
Neon tetraSlender31.211162250
Ram cichlidSlender41.68121737
Zebra/Leopard danioSlender52.0691330
Tiger barbSlender62.4581125
Boesemani RainbowfishSlender114.334614
Sailfin mollyFull176.71225
Kissing gouramiFull259.80124
Fancy goldfish*Full3011.80002
Common goldfish*Full3513.80002
Bala shark/Silver sharkFull3513.80013
Common plecoFull4015.70002
Clown loachFull4015.70002
Sailfin plec/gibbyFull4517.70002

* - Cold-water fish

Frequently Asked Questions about Aquarium Stocking

Q. What about how many fish per gallon rules such "one inch per gallon" or "one centimeter per two litres?"
For starters, the one inch per gallon rule (or 1cm per 2L) only works for tropical freshwater fish; cold and marine fish need more space. Also, tanks are often not filled to capacity; space at the top and displacement by decorations and gravel varies from tank to tank. Nor does it account for the shape of the tank (a consideration as non-standard shapes become popular) or the needs of specific fish species. However, for standard-shaped tropical freshwater aquariums, the one-inch-per-gallon rule gives a similar result as the surface area rule. Regardless of the rule, the adult fish size and body type still needs to be considered--not the juvenile sizes most commonly sold in fish stores.

Q. Why won't more air stones increase the number of fish supported?
It's a misconception that oxygen is added by the bubbles of the air stone. The oxygen added this way is insignificant because the bubbles are present for a very short time. An air stone improves gas exchange by keeping the surface of the water disturbed. This breaks the surface tension as well as the thin mat of dust and biological film that naturally accumulate, which in turn allows oxygen to diffuse properly. Many power filters do the same thing. You should have at least one or the other. As long as something disturbs the surface, adding more air stones will not make a significant difference. This surface area calculator assumes the water surface is being disturbed by filter or air stone.

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 in addition to tank size, such as 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 a fish stocking 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, under-stock.

Q. How much gravel or sand do I need?
To calculate how may pounds (lbs.) of sand or gravel to buy, or for information on what substrate to select, see our "How Much Gravel or Sand Should I Buy?" Aquarium Substrate Calculator.

For more information on fish stocking for beginners, see The Complete Aquarium by Peter W. Scott.