How Many Fish Can I Put In My Aquarium?

So you have setup your Aquarium, and now you are ready to add fish. Here at Aquatix-2u, we have put together a simple yet in-depth guide to help you calculate just how many fish your Aquarium can cater for.

What Affects How Many Fish You Can Keep?

There are a fair amount of permutations that can determine how many fish your aquarium can adequately house. Water quality, oxygen levels and temperature all play a role in keeping your livestock happy and healthy, along with how much room your aquarium can safely offer to new inhabitants.

Water Quality

Adding fish to your aquarium will no doubt have an effect on the water quality, and can potentially cause spikes in harmful pollutants that can pose a risk to your fish. It is important to steadily add fish over a period of months to allow the chemical balance time to adjust with your new additions.

For absolute peace of mind when stocking your aquarium, Aquatix-2u recommends doing regular water tests to check for any spikes or irregularities in the water quality.

For a full in-depth guide to testing your aquarium water, Click Here



Calculating How Many Fish Your Aquarium Can Hold

The most common way of working out how many fish you can keep is to first calculate the water volume of your aquarium. You can easily work this out by multiplying the length, width and depth of the aquarium together (LxWxD). After this, it is recommended to take away 10% to make up for displacement caused by any decoration you may have in the fish tank. Some setups have filtration units, pumps and other devices inside of the aquarium so these may also needs to be considered as well.

Now that you have the volume of your aquarium, next we have to find out the size of the fish you plan on keeping. You can find the typical size of the fish you want to keep by looking up a reliable care sheet. It is important to get the measurements of the fish when they reach adult size, as some fish can begin small but grow to be quite a handful.

Finally, use the table below to work out how many fish you can keep per litre/gallon (Please note this guide uses imperial gallons)



 Fish Type  Inches per Gallon  cm per Litre
 Coldwater  1" per Gallon  2.5cm per 4.55 Litres
 Tropical  1" per 0.5 Gallon  2.5cm per 2.25 Litres
 Marine (Reef)  1" per 4 Gallons  2.5cm per 18 Litres
 Marine (Fish Only)  1" per 2 Gallons  5cm per 9 Litres


For example, if you have a 30-gallon aquarium, it can hold 30" of Coldwater fish, 60" of Tropical fish or 15" of marine fish. Please bear in mind that oval or oddly shaped tanks can make it harder to accurately determine how much room your aquarium has to offer.

Other Things to Bear in Mind:

Overpopulating Your Aquarium

Over filling your Aquarium with fish can become a major problem and should be avoided at all costs. Overpopulation contributes to a number of problems such as water quality deteriorating, filtration problems and aggression over territory or food. It is recommended not to fill your aquarium to its limits but fill to a comfortable amount where you can safely take good care of your livestock.



Different Fish have Different Needs

Depending on the species of fish you want to keep, you should plan accordingly to make sure your aquarium is best suited for them. Some fish are more active than others, so require more room to move about. This is important to remember when thinking about how many fish you can put into your aquarium, as the calculator does not take this into consideration. And lastly, if you are planning on housing several species together make sure that they can live together peacefully.

You can find plenty of articles online explaining the best choice of species that you can confidently keep together.


It is a good idea to research the types of fish you would like to keep before purchasing your Aquarium, this way you can be sure you will be getting the right equipment to take care of your fish.
Here at Aquatix-2u, we offer a wide range of Aquariums, Decoration, Treatments and Test Kits. To view our Aquarium range, Click Here