How Many Fish Per Gallon?

When planning a new aquarium setup, a lot of people follow the standard “one inch per gallon of water” rule, but how accurate is this suggestion? Is that really how you should be planning to stock your tank?

Setting up a thriving aquarium takes a lot of planning, and before you can do so, you have to know how many fish your tank can handle. Overstocking can lead to a lot of problems and drastically affect the health of your fish. It’s important to get it right.

One Inch Per Gallon?

If anything, the “one inch of fish per gallon” rule is more of a rough guideline. It’s a good way to think about stocking a tank because it can keep you from overloading it, but there are exceptions to this rule. You still need to do your research.

This rule means that if you have a ten-gallon tank, you can have 10 inches of fish. But that’s not really the case. For example, a common pleco can grow to about 24 inches, but you can’t keep one in a 25-gallon tank. They need at least 150 gallons.

Let’s look at some of the most common mistakes people make when using the “one inch per gallon” rule.

They forget that the fish aren’t full size

If you buy a fish that’s an inch long in the pet store, that doesn’t mean it’s going to stay that way. Depending on the species, they can get much bigger, in some cases six to eight inches or more.

Do your research before you stock your tank so you know what fish have the potential to get too big for the number of gallons in your tank.

Fish tanks don’t hold as much water as they say they do

A lot of people forget to account for the volume taken up by everything else in the tank. For example, the substrate takes up a decent amount of space, as do the plants and decorations. So, you may have a 10-gallon tank, but it probably only holds about eight of nine gallons of water.

It’s safe to assume you will lose about 10 percent of the total volume. In a 10-gallon tank, that’s about an inch of fish if you’re following the “one inch to one gallon” rule.

Choosing Fish

When stocking your tank, use the “one inch per gallon” rule as a rough guideline, but consider these things when choosing your fish.

Fish are all built differently

Some fish have long, thin bodies while others are round and fat. Angelfish are tall and slim, while fancy goldfish are shorter and squat. This explains why the “one inch per gallon” rule isn’t absolute. You have to consider the actual fish you’re looking at.

Some fish are messy

What you are most concerned with when stocking your tank is how much waste each fish produces. This is important to keep the ammonia, nitrate, and nitrites under control. Too much waste will cause an ammonia spike that could be extremely harmful to your fish.

A good example of this is goldfish. Goldfish are extremely messy, and too many in a tank that’s too small for them will cause a lot of problems.

Read up on the fish you’re thinking of adding to your tank to find out if they produce a lot of waste. If so, you should either understock your tank or invest in a powerful filtration system and plan to do substantial weekly water changes of about 40 to 50 percent.

Some fish need more room to swim

There are species of fish, like corydoras, that like to stick to the bottom of the tank, swimming along the substrate. Something like a danio, on the other hand, likes to school in groups in the middle of the tank with a lot of room to move around.

Active fish that like to swim around need more space than calm fish that stick to one part of the tank. While some of these fish might be the same size, they have different water requirements.

Territorial fish need more space

Compatibility is very important when setting up an aquarium, especially if it’s on the small side. If you have a territorial fish, like a betta, it will make the other fish in the tank miserable, even if you are well within the “one inch for one gallon” rule.

That said, other fish don’t need nearly as much space to call their own, so you can safely keep them together without worrying about leaving extra space. Again, do you research so you know for sure what types of fish can safely and happily live together?

Tank shape factors in

You may not have paid much attention to tank shape before, but it has a lot to do with how many fish you can have per gallon. Some tanks may hold the same amount of water but look drastically different.

For example, say you have two 20-gallon tanks, one that’s tall and one that’s short but wide. The shorter, wider tank has more water surface area. This means that the water is oxygenated more easily, allowing for more fish than the tall tank, even though they hold the same number of gallons.

You can actually use the surface area of the tank to figure out how many fish per gallon the tank can hold. Multiply the width of your tank times the length to get the surface area, then plan for one inch of fish for every 12 square inches.

Again, this isn’t a tried and true rule, more of another rough guideline. You still have to account for all of the other things we mentioned. If you’re planning to keep bigger fish, plan for one inch for every 20 inches of surface area.

Add Your Fish Carefully

The “one inch for one-gallon” rule is a good place to start, but make sure you research the fish you want to add to make sure they don’t produce a lot of waste or need a lot of extra space. Remember, it’s always better to understock a tank than overstock, so playing it safe is always an option.

Even if you’ve planned everything perfectly and know you’re within the parameters for your tank, it’s important to make sure you add the fish to your aquarium the right way. You can’t just add them all at once.

Adding too many fish at the same time can quickly overwhelm the tank. The bioload will rise too quickly, the ammonia will spike, and some of your fish won’t make it. To prevent this, add fish slowly, and check the water regularly to watch for any ammonia or nitrate spikes.

After a few weeks, when things have stabilized, add a few more. Continue this way until your tank has been fully stocked.

Guidelines, Not Rules

Remember that there are no straightforward rules for how many fish per gallon. The “one inch per gallon of water” rule is a good place to start, but it’s not set in stone.

To make sure you plan it right, make sure you take as much time as you need to research the fish you’re thinking about adding to your tank. Make sure they are all compatible with one another and that your tank is big enough to handle them.

Keep an eye on your water parameters, too. If you add more fish and you see spikes that you have a hard time controlling, there are likely too many fish in the tank.