How to figure out how many fish you can have in a tank

One of the most common questions we get is ‘how many fish can I put in my tank?’ And as you can guess, that depends on a few things – from the combination of fish to the size of your aquarium.

To answer this question and to keep things simple, we’re going to break it down into three important things for you to keep in mind. Now be aware, this isn’t a perfect methodology as there are a lot of intricacies involved from species to species of fish – but it’ll help bring your answer into a little bit clearer focus. Let’s jump right in!

Waste load

When fish eat, they produce waste. When you own an aquarium, you’ll have plants and filtration agents inside of it that will help break that waste down. As the waste builds up and outpaces the ability to break it down, the water quality goes down and it can lead to your fish getting sick or even dying. And obviously the more fish you have, the more waste will quickly accumulate. Because of that, it’s important to not overload your aquarium with fish. We call this ‘waste load’ – or essentially the threshold where the fish in it will produce too much waste to keep a tank clean.

So how do you minimize waste load? The answer is your filtration system. Beneficial bacteria that lives in things like bioballs will help break down the toxic compounds. Make sure you follow your aquarium cycling instructions to put yourself in the best position to have a thriving, healthy environment for your fish to live in. Even consider some nitrifying bacteria to help jump start the process.

Lastly, be aware of the kind of food you’re feeding to your fish. Low quality food breaks apart easily and can create more waste. Not to gross you out, but fish aren’t unlike humans in that regard. Healthier food means their bodies use more of it and produce less waste. Bad food is just the fish version of junk food. It’ll keep them going, but it produces a lot more waste. Talk to your local pet store or vendor about some ‘clean’ fish food for your fish. Not only will it produce less waste, but it’ll be healthier too.

Swimming space

The conventional wisdom is that you keep 1 inch of fish for every gallon of water. And then of course, you apply the theory across the board. So if you have a 10 gallon fish tank – then you have 10 inches worth of fish length to put in there. Get a bigger fish that’s 2-3 inches long and well, that counts against your 10 inch rule.

In addition, before you buy a fish, make sure to research tier minimum tank requirements. Just because they’re a certain size doesn’t mean they don’t need more room. It might seem silly, but think of fish like dog breeds. Sure, dogs are all dogs – but the kind of care, space and activity a bulldog needs as opposed to a golden retriever is like night and day. Fish aren’t any different. Some are very active and need a lot of swimming space. Others are fairly sedentary and as long as they have a small patch to hang out in, they’re good to go.

So before you purchase a fish, do your homework!

Aggression

It’s a jungle out there and the final factor you’ll want to consider is how aggressive your fish are. Fish behave differently and this goes along with what we talked about above. Sometimes size and space doesn’t matter – because some fish just wanna fight. Be sure that if you’re looking to add population to your tank, that you figure out what your new neighbors behavior patterns are first.

Fish interact with each other differently. Some species don’t like others and some species really compliment each other well. Just like above – make sure you ask questions and do some research into which fish will thrive in the ecosystem you’ve created inside your tank.

And now that we think of it, that’s an important thing to emphasize. The biggest thing is to remember that your tank is an ecosystem – but one that can change over time. Start small and build it up. Don’t buy a bunch of fish all at once. You may decide over time that you want something different from what you already have. Take your time, learn on your own and just like with any other hobby you have – do your homework! Good luck!