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The One-Inch-Per-Gallon Rule Explained

Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.

The One-Inch-Per-Gallon Rule for Aquarium Fish

The One-Inch-Per-Gallon Rule for Aquarium Fish

How Many Fish Per Gallon?

The one-inch-per-gallon rule is an easy way to calculate how many fish you can stock in your aquarium. It considers the size of the aquarium and recommends one inch of adult fish for every gallon of water.

Therefore, if you are wondering how many fish you can have in a ten-gallon tank, this rule tells us you can stock ten fish that grow to an adult length of one inch each. Or, you can choose five two-inch fish. Or two three-inch fish and one four-inch fish.

You get the idea. This simple math makes choosing fish for an aquarium super easy, even for beginning fish keepers. The basic rule of thumb has been floating around for decades, and countless aquarium owners have relied on it to make decisions.

There is only one problem here: This is horrible advice. Most times, the one-inch-per-gallon rule oversimplifies the tank stocking process and can lead to dangerous conditions in your aquarium.

In this article, you will learn the truth behind this oft-repeated rule of thumb. As always, my advice is to learn as much as you can about the fish you intend to stock and remember that any information you read here is based on my experience and opinions. The more you can learn, the better.

One-Inch-Per-Gallon Rule Issues

The examples I gave above make it seem like our rule of thumb has some merit, but once you think about it a little this advice starts to fall apart.

According to this rule, you could put a 10-inch fish in a 10-gallon tank. Or how about two 27-inch fish in a 55-gallon tank? Or four 12-inch fish in a 55-gallon aquarium?

Does any of that sound right? I sure hope not. Those are extreme examples of big fish that are all too large for those tanks. The math might add up, but it makes no sense.

Large fish require a certain amount of planning and expertise if you wish to keep them successfully, but they are only one problem here. Behavior is another. Would you put a 12-inch oscar in a 20-gallon tank with a bunch of one-inch fish and expect the little fish to survive?

Likewise, you could justify keeping a pair of gouramis in a 10-gallon tank based on the one-inch-per-gallon rule, but their behaviors might make your aquarium a war zone.

Aquarium fish require individual consideration when choosing their tank and tank mates. No rule of thumb can help you here. The only correct method is to learn as much as you can about the fish you intend to stock before you bring them home, so you have to make sure you have the right environment for them.

This is certainly more work, but it helps ensure you have not only the right number of fish in your tank but tank mates who will get along.

Complex fish require special considerations.

Complex fish require special considerations.

How to Choose Fish for Your Aquarium

There is a better way. Here are a few things to think about when choosing fish for your aquarium. Remember, you’ll need to do a little research on the individual species you are interested in. Learning about the fish you intend to keep so you can best care for them is part of the aquarium hobby and there is no way around it.

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Overall Bioload

The bioload is a fancy way of talking about the waste your fish put into their environment, and how well the natural processes in your aquarium can deal with it. Bacterial colonies live in aquariums, and they help break down waste and turn harmful chemicals into less harmful chemicals.

The number and size of your fish influence this process. Healthy microbes in overstocked tanks have a tough time keeping up, and the result is a toxic environment for your fish. Over-filtering and keeping live plants can help. Smaller tanks are tougher to manage than larger ones, but any time you have too many fish, you are asking for trouble.

You don't have to guess about the health of your tank water. You can and should use a freshwater test kit to monitor your water parameters so you are aware of any creeping issues in your aquarium. I suggest the API Freshwater Master Test Kit, as I've always found it easy to use and accurate. But you can look around and see which kit you think is best.

Schooling Behaviors

Some fish need to live in schools to feel safe. This means you’ll want at least six of them. This applies to many tetras such as neons, but also to fish you might not expect, such as cory catfish. Fish that need to live in groups will get stressed when kept alone or in small numbers, and stress is one of the main reasons aquarium fish die before their time.

What this means is you might need a bigger tank than you think if you intend to keep schooling fish. Six cories in a 10-gallon tank don’t leave a lot of room for other inhabitants.

Semi-Aggressive Behaviors

Semi-aggressive is a term you’ll see on the tank labels in pet stores. It makes it sound like the fish is a little mean, but they'll be fine as long as other fish don’t mess with them too much. What it really means is, that in the wrong environment, the fish might be very aggressive. This includes territorial behaviors, predatory behaviors, and cases where fish just need a little more space than others.

For example, in the right environment, freshwater angelfish are very peaceful. Put them in the wrong situation and they can be demons. Put them in with small fish and you’ll be setting them up for a pleasant lunch. Tank size and tank mates are important when stocking angelfish.

Be especially careful when stocking semi-aggressive fish and make sure you aren’t putting them in a situation that will trigger their dangerous behaviors.

Angelfish are semi-aggressive fish that require planning and thought.

Angelfish are semi-aggressive fish that require planning and thought.

Social Behaviors

Some fish exhibit social behaviors that require consideration when stocking them. For example, livebearers such as guppies, swordtails, and platies should be stocked at a ratio of at least 2 females for each male. This is because males hoping to mate can relentlessly harass a single female fish. More females mean the stress is spread to multiple fish, meaning less stress for any one female.

Some fish are best kept in pairs, whereas others, such as bettas, are usually better off alone. Once again, there is no substitute for research here.

Space in the Water Column

Besides making sure you don’t have so many fish that you tip the tank’s bioload into dangerous territory, you want to make sure your fish have enough physical room. Peaceful fish that feel overcrowded can become nippy, or outright aggressive. Or, they may simply become so stressed that they get sick.

Think about the specific regions of the water column where your fish will live. Catfish and plecos tend to stay on the bottom or on the tank glass. Fish like neons tetras typically stick to the middle of the water column. Some fish, like platies and guppies, like to swim all over the place.

Having a basic idea of where your fish will live in your tank will help keep you ensure everyone has enough space to thrive.

Good Luck Choosing the Best Fish for Your Tank!

You can see the trouble that relying on a simple rule of thumb to guide your stocking choices can cause. When you are in doubt, it is always better to understock your tank.

Fish are complicated creatures, and their needs vary by species. Doing the necessary research to choose fish wisely is a little more work, but it gives you a much better chance of a happy outcome.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

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