How Many Fish Should Be Kept in an Aquarium?

-- By Carol

Most aquarium owners wonder how many fish they can keep in their aquarium.

While it might be very tempting to buy a good variety of colorful fish, overcrowding your aquarium is, in fact, risky for the aquatic life inside it.

Your aquarium has a real underwater ecosystem that depends greatly on the chemical composition and pH of the water in the aquarium.

It would be much better if all aquariums come with a stocking chart but the problem is they don’t.

To determine how many fish you should keep in an aquarium, there are a number of things that you need to consider.

Fortunately, you can easily calculate the number of fish you should keep in any type of aquarium with just a few simple techniques and rules.

Table of Contents

The One Inch per Gallon Rule

The total number of fish to stock in your aquarium depends greatly on the size of your aquarium.

There is no actual way to determine the exact number of fish that you should keep in an aquarium but the most commonly known rule for stocking fish in an aquarium is the one inch of fish per gallon rule.

It might sound very simple and easy to follow but it can be very tricky for fish owners in reality.

The calculation can give a quick estimate but has some downsides to it. It neither considers modern filtration systems nor other factors like the amount of light, types of fish, etc.

Like any other living being, fish also come in different shapes and sizes.

Stocking your aquarium with small or large-bodied fish is not the same in any case.

Additionally, fishes that are larger in size create more waste than smaller ones and, therefore, need more water volume to survive.

Factors to Determine How many Fish to Keep in an Aquarium

The following factors can help you determine the number of fish you should keep in your aquarium:

The Capacity of your Aquarium

The first step to determine how many fish you can keep in your aquarium is to calculate the size or capacity of your aquarium.

Aquariums are generally measured in liters. When you purchase the aquarium, you get to know about its exact size.

If you don’t know your aquarium size due to any reason, you can very easily calculate it on your own.

You will have to measure the length, width, and height of your aquarium in centimeters, multiply them together to get the volume, and then divide it by 1000.

You should keep in mind that this is only a rough estimate as the actual capacity would be even lesser due to the thickness of the glass, decorations, and other objects in the aquarium.

The Water Level of Your Aquarium

Another important factor that determines how many fish you should keep in the aquarium is the water level.

You have to maintain the right level of water to accurately determine the number of fish that can survive in it.

If the water level in your aquarium is very low, you will not be able to keep as many fish as the aquarium could house.

The water level should ideally be one inch above the top of the water filter.

Adding different objects like driftwood, rocks, shells, etc displaces water which means it decreases the water level in the aquarium.

If you have a lot of decorations for your aquarium, you cannot keep as many fish as an aquarium of the same size with lesser decorations can hold.

One Inch per Gallon or Surface Area Rule

The two most commonly used rules for determining the stocking of fish are the one inch per gallon or surface area rule.

One inch fish per gallon of water rule works best for small community fish. If you are to house bigger fish in your aquarium, this rule might not be suitable for accurate calculation.

Another rule is the surface area rule.

Determining the surface area of the water is important because all gaseous exchanges take place on the surface of the water to provide the fish with suitable living conditions.

A larger surface area ensures greater oxygen exchange for the fish in the aquarium.

An aquarium that is bigger in height but has the same surface area as a small aquarium cannot hold more fish even though it has more water than the smaller aquarium.

You can thus use this rule for stocking fish all while ensuring that the exchange of oxygen is not compromised.

Types of Fish for Your Aquarium

Another important factor to determine the number of fish you can keep in your aquarium is the type of fish you choose.

Different people have different preferences; some might want lots of small fishes while others might prefer a semi-aggressive community of larger fish.

It is important to do some research before choosing a type of fish for your aquarium.

Learn as much as you can about their compatibility and the maximum size they reach.

More aggressive fishes need more space while some smaller ones might live peacefully in a larger community.

It is, therefore, important to know all about the type of fish you want to add to your aquarium and then determine how many you can keep.

Stocking Levels

While one inch of adult fish per gallon of water is a suitable rule for smaller fish, it can become trickier for larger fish.

In an ideal condition where none of your fish is too big for your aquarium or not inappropriate to keep with other fish, you can start off by calculating one inch for 7.5 liters of water.

This can give you a balanced estimate to determine the stocking level for your aquarium.

You will also have to determine which types of fish swim at the top, middle, or bottom.

This can help you manage the stocking level in a better way by balancing the right number of fish for your aquarium.

Oxygen Needs of Your Fish

Like many other things, the oxygen needs of different types of fish also vary.

Larger-bodied fish need more oxygen than narrow-bodied fish.

If you are planning to keep larger fish in your tank, they would need more oxygen as well as more space in the aquarium.

To properly oxygenate the water in your aquarium, we recommend you add air stones and a filter to your aquarium.

When determining the stocking level of your aquarium, you should always consider the adult size or weight of the fish.

This helps you consider the different oxygen needs of the fish. It is an important factor to consider in determining how many fish you can keep in your aquarium.

Filtration of Your Aquarium

Filtration of your aquarium also plays a key role in determining how many fish you can keep in your aquarium.

The aquarium filter should be able to run all the water in your aquarium four times through it every hour.

This simply means that if you have a 10-gallon aquarium, the filter should be able to run 40 gallons of water every hour.

A filter exceeding this limit is absolutely no problem while a filter not meeting this criterion can affect the number of fishes you can keep in your aquarium.

Some Bonus Tips for Stocking your Aquarium

For normal situations and smaller fish, the commonly used inch per gallon rule is quite suitable to calculate the number of fish you can keep in your aquarium.

If you are to calculate the number of fish with this rule, you should always consider the net gallons of water and also consider the adult size, shape, and needs of the fish.

If your aquarium is not of standard size, it is better to work with the surface area rule.

Regardless of the rule, you are to use, you should always do some thorough research about the type of fish you have to keep in your aquarium.

It is always better to stay under the limit rather than going over.

That said, do not stock your aquarium full to its capacity at a time. You should not add more than 25% of the total stocking level of fish to your aquarium.

The by-product of the fish is broken down by beneficial bacteria in the aquarium and these bacteria colonies need time to adjust to the environment and changes in the bio-load.

By adding a few fish at a time to your aquarium, you give enough time to the bacteria colonies to grow and break down the fish waste.

Reading through these important factors, one can easily understand that there is no fixed number of fish that you can keep in your aquarium.

It all comes down to a lot of factors before you safely calculate the right number of fish for your aquarium with the help of the inch per gallon or surface area rule.

The best way is to determine the capacity of your aquarium, do some research about the type of fish you wish to keep in the aquarium, and then introduce a small amount of fish to prevent any problems.

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