How Many Fish Can I Put In My Tank?

Overstocked AquariumOne of the most common questions asked when setting up a fish tank is, “How many fish can I put in my fish tank?”.  Although it seems like a simple question, there really isn’t a simple answer. There is a lot more involved than most people realize when stocking a fish tank. While many people have probably heard of the inch of fish per gallon rule, this really doesn’t take into account all the factors.

Inch of Fish Rule

Even though I don’t recommend using the inch of fish per gallon of water rule, I wanted to go into a little more detail of what it is and why it doesn’t work. This basic rule essentially says you can add X inches of fish for X gallons of water. So following this rule, if you had a 10 gallon aquarium, you could have 10 one inch long fish. One big problem with this rule is that a 10 gallon tank probably doesn’t have 10 gallons of water in it after you add gravel, equipment and decorations. Another problem is this doesn’t take into effect the most important thing when stocking fish, gas exchange.

The gas exchange rate in your fish tank depends on a number of different factors; however the biggest factor is the surface area of the top of the fish tank. Even though fish live underwater, they breath dissolved oxygen through their gills and expel carbon dioxide. If your fish tank is overcrowded, the oxygen will be depleted from the water and the carbon dioxide will not escape from the water fast enough. The surface area of the open part of the tank greatly affects how many fish you can have. If you have a tall narrow tank, you would be able to keep less fish than a long wide tank.

The Right Way to Stock a Fish Tank

The absolute definitive way to stock a fish tank would be to determine the biomass of each fish you plan on putting into the fish tank and calculating out the gas exchange rate of your aquarium. Unfortunately doing this would be extremely difficult because each fish has a different biomass and there are so many factors in the gas exchange rate. Having live plants vs. not having them will affect how many fish you could have in your aquarium.

So you might be asking yourself at this point, how do you know how many fish you can put in your fish tank. Well, I found a really handy aquarium stocking calculator over at To use the calculator you just need to enter in your tank’s length and width and the calculator will estimate how many inches of fish you can have in your aquarium. (Here’s a list of common aquarium dimensions as well) I particularly like this calculator verse some of the others I’ve seen because it breaks it down very nicely and tells you how many tropical freshwater fish, cold water fish, or saltwater fish you could have in your fish tank. Obviously, this is just an estimation and you would want to make sure that whatever fish you add to your aquarium is appropriate for your tank. So you wouldn’t want to add two 4 inch Oscars to a 10 gallon tank even though the calculator says 9 inches would be appropriate.

No matter what method you use to stock your fish tank, it is always better to understock than overstock. It’s also important to note that the fish you buy will most likely grow, so when you add your fish you should be using it’s adult length in your calculation of total inches of fish. Otherwise, you will be stuck either needing to get rid of the fish, set up a separate fish tank, or get a bigger fish tank. The last two options aren’t the worst thing in the world, but still you want to make sure your fish won’t outgrow your aquarium.

Thanks for reading and learning how to properly stock a fish tank. If you found this helpful and would like to know other tips and tricks, including the secrets many fish stores won’t tell you, please join my mailing list.


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