How Many Fish Per Gallon Of Water In Aquarium?

how many fish per gallon

When owning fish, you want them to be as comfortable in their tank as they possibly can be, when you own more than one fish, the question of their comfort becomes a question of overcrowding.

The rule that has been used throughout the time humans have kept fish as pets is one inch of fish per gallon of water.

However, this is more of a myth than a guide, because it makes no sense.

One inch of Guppy is not the same as one inch of Koi.

Different species of fish have very different requirements when it comes to space requirements, dietary habits, and filtration needs.

Using the one inch of fish per gallon of water rule can therefore be harmful to the species of fish you are keeping.

In this article, we will explore the actual size aquarium you need for keeping fish and the factors that go into choosing the size of aquarium you need. 

What is the Correct Fish per Gallon Ratio?

The simple answer is, there isn’t one.

The space you have for the amount of fish you are keeping will all depend on the species of fish you want to keep, the decision on what fish you want to keep should always be made prior to buying any puppies for keeping your fish, including the aquarium you are going to keep the fish in.

If you are thinking of purchasing more than one species of fish to cohabit in the same aquarium then make sure you do your research prior to buying the fish, do they get along?

Will they eat each other?

Do they have the same water quality tolerances?

Do they have the same diet?

These are all crucial questions that you need to ask yourself before putting two separate species of fish in the same habitat.

You should also know exactly what your fish needs even if you are only taking care of one species of fish.

Each species of fish will be different in its body type and territory, this will affect the size habitat you keep them in.

It goes without saying that the size of a fish is the most important thing to take into account when deciding on the size of its tank, but what is more important is to know how big the fish is going to get.

There is a common misconception that a fish will only grow to the size of its tank, this is false, if you feed a fish a high-quality diet, it will keep growing and can outgrow its tank.

Always research how big your fish can get and base your tank size around that.

When you plan your habitat size around the size that your fish could grow, you won’t find yourself needing to change your fish’s habitat to make them more comfortable.

Always go for bigger than you need and more filtration than you need if you are worried the habitat is too small.

You will be surprised at how big your habitat will need to be if you look at the size that your species could grow, for example, Koi may start out around the size of goldfish, but it is recommended that you give them a habitat with 250 gallons of water per fish!

This is because Koi fish can grow very large.

how many fish per gallon

Water Volume, Tank Dimension, and Other Tank tips

Another reason why the one inch of fish per gallon of water rule is a terrible rule to follow is that people will not realize how much less the true water volume of their tank is than what they realize.

Although your tank may say it is a ten-gallon tank, when fish, decorations, and substrate are added, they displace the water, meaning that water cannot exist in the same space as these things.

Therefore the tank can hold less water than it says it can, as the displaced water has to be substituted somewhere else.

This means that your ten-gallon tank will be more like eight or nine gallons by the time decoration and fish are added.

It may not seem like a big deal but that is up to 20% less water in your tank to buffer chemistry changes, disperse CO2, and hold oxygen, which means it will be able to house less fish.

You should keep this in mind when deciding the dimensions of the tank you want to buy for your fish.

If you want to keep large fish such as Koi or Oscar then the size of your tank will be very important.

It is recommended that an absolute minimum size tank for large species of fish hold 55 gallons of water.

However, when you take into account the water dispersed by the large fish and other decorations in the tank, you will need a bigger minimum tank.

Therefore we recommend that you have a minimum of 75 gallons for large fish habitats.

You also need to take into account fish species that like to hang out in schools and swim swiftly over large areas.

It is no use using the one inch of fish per gallon of water rule here as the size of the school acts like a huge fish that will need a good amount of space to swim around in.

In these cases, it may be a good idea to get a bigger tank than you usually would for the smaller fish.

Final Thoughts

The amount of water and the size of habitat you need for your fish will depend on various different factors.

Most importantly, the species of fish that you have, make sure that you research the species of fish you are planning to purchase extensively before looking at habitats that you need to keep the fish in.

It is also important to construct the habitat around the size that your fish can grow instead of the size they are.

It is always best to go bigger if you are worried about the size of your tank, fish will enjoy the room they are given and will thrive because of it.

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