How Many Fish Per Gallon Can You Keep in Aquarium – Stock Level Calculator

How many fish can you keep in an aquarium?“, is the first question that people encounter with when they get the idea of owning an aquarium. Though there are certain guidelines about the number of fish to be put, the scientists claim that the exact rules can’t be set apart from the basic two thumb rules because stocking the aquarium with fish is a function of many factors out of which the surface area and the volume of the tank play important roles. The surface area of a fish tank influences the oxygen levels of the tank while the volume plays a bigger role in the pollution of the water by the excretion of the waste.

How Many Fish Can You Keep In Your Aquarium

How Many Fish Can You Keep In Your Aquarium

Many other things that come into the picture while you talk about placing fish in the fish tank include:

  1. Shape of the aquarium
  2. Agitation of the surface
  3. The type of filtration system that has been embedded
  4. The water pH levels and the temperature
  5. Maintenance regime of the fish tank
  6. The type of fish that is kept inside.


A thing to consider when you’re a beginner in the fish keeping business is that it is not recommended to stock the maximum fish, i.e., 1 inch of fish for one gallon at your first go. For example, if you’re having a 55 gallon fish tank make sure that you are stocking it with around 40 inches of fish at first. Also, the 40 fish shouldn’t just be placed inside at a time but should be arranged over a period of around 6 months or more by giving the tank the time to mature gradually.

How Many Fish Per Gallon Can I Place In My Fish Tank?

With all the caveats placed, the common guidelines that are to be followed while stocking the fish are:

  1. 1 cm of fish per 1 liter/1 inch of fish per 1 gallon.
  2. If you’re using cold water fish, 2.5 cm for every 4.5 liters of water.
  3. If you’re using tropical fish, 2.5 cm/2.5 liters and 1 inch/0.5 gallons.
  4. If you’re using Marine reef fish, 1 inch/4 gallons, 2.5cm/18 liters.


The advantage of the inch per gallon rule is that it is very easy to understand and calculate as most people buy the fish tank based on the volume. For example, if you have a 55 gallon fish tank; you can place around 14 inches of Marine reef fish in it.

Also, by buying a bigger tank, you’re diluting the water and are reducing the impact of the pollution of the water. Bigger water volumes can carry more fish and as well as the fish waste but however they don’t create a mess when it comes to the excreted waste because of the water volume. The only disadvantage of the inch per gallon rule is that it doesn’t consider the shape and the surface area of the water.

Surface Area – Important Factor While Stocking Aquarium

For example, if you have two 55 gallon tanks with the same volume but different shapes; i.e, one is a rectangular tank and the other is a tall and narrow tank; by following this rule both the tanks should be accommodated with the same number of fish as they will have the equal carrying capacity. But in reality, the shallow tank has more surface area and less width and therefore, the level zoning is important in such cases.

Why do we have to consider the surface area, if you ask? The surface area is responsible for the gas exchange including the oxygen levels. However, most of the people are not exactly aware of the surface area of the fish tank as they don’t know that it’s a simple multiplication of the length and width of the tank and can be done in a minute or less.

Once you known the surface area along with the relevant issues of fish keeping, you can determine whether you can keep more fish inside the tank or not. The third issue that is to be considered is the displacement of the water inside the tank and the decoration.

Even if your tank has a theoretical volume of 55 gallons, you have to exclude a 10-20% of the tank for the decoration like the bogwood, driftwood, live rocks, plants and gravel. If you’re planning for heavily furnished fish tank, make sure that it doesn’t affect the inside movement of the water.

Calculating Fish Numbers to Put in Your Tank

Different fish require a different amount of space in your fish tank. Fishes like Danios or Tangs require more space for swimming than the static and calmer fish like the Neon Tetras. Therefore, based on the type of fish; the territory is required.

For example, you can only keep a couple of dwarf cichlids in two-foot space irrespective of the volume of the tank. Bigger fish need more space than the little fish, even when they are equivalent in inches. A long catfish might require more space than the danios or tetras. Also, the messier eaters are to be given more space while compared with the introvert fish like the shrimps and snails.

Calculating the biological load before placing the fish will help you in giving a clear idea about the inclusive carrying capacity, the required water quality, the interval period between water changes, type of filtration that is to be used, what kind of water circulation is required et.al.

Aquarium Stock Level Calculator

Aquarium stock level calculator is a tool you can use online for getting approximate idea for stocking fish in your fish tank. No tool is perfect, but yes, you can get a rough estimate numbers of fish you should keep in an aquarium.

Here is the aquarium stock level calculator tool: aquarium stock level calculator

Finally, How Many Fish is Ideal Number?

Concluding, it is always better to have few healthy fish with adequate space required for swimming, roaming, hiding and for their messier eating, their waste than to stock maximum fish and prone them to molestation. Cramming is not recommended until unless you know about the fish, their aggressive levels, the fighting behavior and also the diseases they can suffer with.

Know: Freshwater Aquarium vs Saltwater Aquarium

These are just a few things that are to be considered as an erring of caution and are best policies to use while you’re stocking your aquarium. While an understocked fish tank is easy to maintain than an overstocked one, you can decorate the under stocked tank with many decorations that are being invented these days. What are you waiting for? Stock your tank, already.

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