How Many Fish in a Tank Calculator? (Fish Tank Capacity)

how many fish in a tank calculator

An overstocked aquarium can cause problems such as territorial fighting and water pollution, which can harm your fish’s life. Therefore, fishkeepers need to calculate the number of fish their tank can hold inside to maintain good living conditions for all the tank creatures.

Indeed, many people search for ‘how many fish in a tank calculator to quickly get the number of fish for their tank capacity. However, multiple additional factors influence the final decision on the number of fish. Read on to know.

How Many Fish in an Aquarium


The good old way to calculate the number of fish per gallon is the 1 inch fish per gallon rule, primarily applicable to fish from 1 – 1.5 inches long. Following this fish per gallon calculator, a 5-gallon aquarium can hold up to 5 inches of fish.

But, this fish-to-tank ratio is not the best tool for exact calculation as the fish types, tank design, and other factors are not considered.

Therefore, many aquarists prefer the fish stock calculator via this tool below to figure out the number of fish they can put in a tank.

The fish calculator will solve the math for you when you enter the maximum length and full-body length of your fish, as well as the tank dimensions.

What Does the Tank Stocking Calculator Result Mean?

The stocking level calculator considers the aquarium surface for oxygen diffusion rather than calculating the fish amount based on gallons of water. There is approximately one inch of adult fish for every 12 square inches of surface area.

This calculator works well as it applies to fish of various sizes. Since the calculator considers the specific needs of different fish for oxygen, it can ensure good living conditions for various fish.

The surface area of the aquarium can estimate the percent of oxygen dissolved in water, which is the main factor in determining the number of fish that can live in it.

However, here’re some things to note down when using the fish tank stocking calculator:

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  • Research the oxygen needs of the fish before calculating.
  • Maintain the aquarium conditions well to ensure the optimal environment that this calculator considers.
  • Consider the fish’s temperament, as even when the oxygen needs are met; they can be aggressive and harm the tank mates.
  • This calculator does not consider plants, and additional shrimps or snails, so you need to consider these side factors.
  • Add new fish gradually.
  • Remember, using filtration and air stones for surface agitation does not give you a chance to have more fish in your tank.


Different Sizes of Tank

5 Gallon Tank

A 5-gallon tank can hold up to 5 inches of slender fish. On average, your little tank can have 2-4 fish as long as each fish species is smaller than 1 inch.

Chili Rasbora, Scarlet Badis, Ember Tetra, and Celestial Pearl Danio are also some best fish suggestions for a 5-gallon aquarium.

Adult fish growing larger than 1 inch are not advisable as they can produce large amounts of waste that cause fish to die.

10-gallon tank


Using the fish stock calculator, there can be either 7-10 tetras or up to 8 Danios in a 10-gallon tank. Betta Fish, Guppies, Dwarf Gourami, or Golden Dwarf Barbs are good freshwater fish suggestions for this small aquarium.

Generally, a 10-gallon fish tank can house up to 10 inches of slender fish or just 3-4 inches of large fish. However, large fish are not recommended as they can be crowded and produce a large amount of waste.

20-gallon tank


This tank size can generally hold up to 20 slim-bodies fish that should be fewer than 1-inch in size. In a saltwater tank, you can keep either 2 Clownfish or 3 Damsel fish.

Fish larger than 4.5 inches are not advisable in 20-gallon tanks as they may reach the tank’s carrying capacity limits.

29 Gallon Tank

As usual, there can be more than 20 super little fish less than 1-inch that can be in a 29 gallon tank. But we are sure you want to keep various fish species rather than one. Some best fish suggestions for your aquarium include Guppies, Neon Tetras, Bristlenose plecos, and Platies.

29-gallon is a good medium tank most suitable for a medium-sized fish community. Large-bodied fish are a little bit crowded, which can cause territorial behavior and water pollution.

30 Gallon Tank

A 30-gallon fish tank closely equals 120 litre tank, which is also among the recommended size for starters.

This tank can house a variety of fish, from small to medium-sized fish species. If you prefer large fish, make sure they are less than 5 inches in length when it’s fully grown.

Tropical fish such as Neon Tetra, Cory Catfish, and Mollies are desirable species to add.

36 Gallon Tank

A 36-gallon bowfront has enough space to house most medium and a few large-bodied fish under 6 inches.

Tetras, Rasboras, African cichlids, and Fancy Goldfish are some best fish suggestions that you can put in a 36 gallon tank.

Having some shrimp and algae eaters also make your aquarium more impressive, while algae eaters are also helpful in maintaining the tank.

55 Gallon Tank

One of the most common large tanks is a 55-gallon tank that can support any size of fish you want. Basically, a 55-gallon tank can hold up to more than 30 Ember Tetras and 2 Male Dwarf Gourami. Other fish ideas include Fancy Goldfish, Mollies, and Angelfish.

Since a 55-gallon tank is a good size for raising many different fish species, you need to start paying attention to fish compatibility as not all of them can get along and stay peaceful.

Size of Fish

Most of the fish that you buy in the fish store are juveniles. Red Tail Sharks, for instance, may only be 2-inches in the pet store, but they can grow up to five inches within a year.

All the rules or fish tank capacity calculators mainly apply to the adult size of fish. Therefore when calculating how many fish your tank can keep, you should double its size or use the full-grown size of the fish for the accurate number.

Example Types of Tropical Fish in Aquariums


The table below shows the maximum stock levels of each fish species. But this table should be used as a reference only. Before adding it to your tank, you need to research each fish’s behavior, living conditions, and compatibility with other fish.

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Fish speciesType of bodyMax length (inch) Number of fish
10-gallon aquarium20-gallon aquarium29-gallon aquarium30-gallon aquarium36-gallon aquarium55-gallon aquarium
Neon tetraSlender torpedo-shaped body1.2471624253040
Tiger barbArrow-shaped slender body2.424812131522
Fancy goldfishFull120001112




What about how many fish per gallon rules such “one inch per gallon” or “one centimeter per two litres?”

In general, the 1-inch-per-gallon water or 1cm of fish length per 2L rule is only effective for standard freshwater tropical fish tanks that can give a similar outcome just like the surface area calculator.

You should know that this rule doesn’t consider other factors including displacement of water.

When calculating the number of fish, no matter what rules or calculators you use, you need to notice the adult fish’s length as well as body shape.

As you see, not only gallons but many people also calculate the number of fish per litre. You can use both units to work out fish numbers.

Why won’t more air stones increase the number of fish supported?

If you think that oxygen in the fish tank is provided by the airstone, it’s a misconception.

It’s true that air stone has a positive impact on increasing gas exchange by disturbing the water’s surface. But this method is insignificant as the bubbler is only present in the water for a short time.

So having more airstones doesn’t make your tank hold more fish either.

Is there a more precise way to perform a fish stocking calculation?

In theory, the most precise calculation will require the tank’s gas exchange rate and the fish’s biomass.

Knowing a fully grown-up fish’s weight is a prerequisite to finding out the fish biomass. This is challenging! So it is advisable to use an aquarium stock calculator.

In addition, the number of fish in an aquarium also depends on the size of the tank. The fish tank size chart can be helpful in choosing the most suitable aquarium size.

How much gravel or sand do I need?

With small to medium aquariums, it is recommended 2 -3 inches of gravel or 1 inch of sand in the bottom of your aquarium tank, while a larger fish tank may require 3- 4 inches of gravel or 3 inches of sand.


How many fish in a tank calculator? You may get your answer after reading the whole article.

A beginner can use the rule of thumb, while an experienced person can have various ways to work out the number of fish in a particular size of the tank, such as using a tank stocking calculator.

Last but not least, you can use traditional or calculator rules. On top of that, you have to know the adult size and pay attention to other things like natural behaviors, type of fish, and environmental factors.