How Many Fish Can You Put In A Tank At Once?

Optimally stock your aquarium with tips from The iFISH Store!


If you’re a beginner aquarist, one of the most common questions you may have is, how many fish can I put in my tank? And, unfortunately, there is no quick guide on the side to reference.

Because there is so much confusion around properly stocking an aquarium, overstocking a tank is a typical mishap amongst new fish owners. So, how many fish can you really keep?

The iFISH Store provides the leading selection of freshwater fish online, complete with our live arrival guarantee. Learn more about how to optimally stock your aquarium in today’s post.

infographic about stocking a fish tank properly

The One Inch Rule

The one inch rule — one inch of fish per gallon of water — is a starting point and a good rough estimate, but it certainly isn’t the end-all-be-all point of reference because it often doesn’t account for factors such as live plants, modern filtration, lighting and the variety of fish you want to keep.

Even novice aquarists know that fish are not all the same shape or size, so stocking a ten-gallon tank with 10 inches of goldfish will be much different than 10 inches of tetra. You also have to take into account that larger fish create more waste, thus need more water volume as a result.

Fish also need enough room to move and swim about, so even though a certain type of fish may in theory look great for your tank size, it may be too small for active species and schooling fish.

It is also known that schooling fish need to be kept in multiples, which leads to more space in a tank — adding only a couple of schooling fish is very stressful and often leads to a reduced lifespan.

Factor In Growth

It’s very likely that when your fish arrive, they won’t be fully grown, so factoring in their full adult size must be used when calculating your tank for stocking. The best way to determine how large a fish will grow is do a little research, and talk with our fish experts at The iFISH Store.

The Real Tank Size

While an aquarium might be labeled as a 10-gallon, you can’t jump to the conclusion that it will hold 10-gallons of water.

Yes, it is true that a 10-gallon tank can hold 10 gallons of water, but that is only true when you add just water. As we know, a fish tank is much more than just water and fish. Factoring in gravel, rocks, plants, and decorations reduces the overall water volume. Typically the water volume is 15% less than the size of the actual aquarium.

At the end of the day, if you’re stocking your freshwater tank with cichlids or goldfish, one inch of fish per two gallons of water is much more of a realistic number to go by.

Considering Surface Area

How your tank is shaped — the surface area — will greatly affect how many fish can be stocked. A tank that is tall and slender may hold the same amount of gallons, but the surface area is going to differ greatly compared to a tank that is short and wide.

You can find the surface area by multiplying the width times the length of the tank. A general water surface area rule is the aquarium can be stocked with one inch of fish for every 12 square inches of surface area.

Again, this measure is a starting point, and is great for unconventionally shaped tanks.

Beginning To Stock Your Tank

As you take into consideration the size, shape, and types of fish you’re going to stock in your tank, it’s important to not fully stock your tank all at once. A rough guide is to only stock 25% or less of the total volume of fish at once.

It’s also very important to consider your filtration as you begin to stock your tank. Because fish waste is toxic and as the beneficial bacteria colonies are being established, you’ll need a filter that can run four times the total water volume of the tank in one hour. For example, a 10-gallon tank requires a filtration system rated at 40 gallons per hour.

Ready To Stock your Tank?

Now that you have a better idea of how many fish can be safely and optimally stocked, it’s time to fill your aquarium with beautiful, freshwater fish.

From cichlids and discus to goldfish and betta, your freshwater aquarium will be healthy and vibrant when you follow the one- to two-inch fish rule per gallon and the surface area suggestion of one inch of fish per 12 square inches of surface area.

Browse our fish, or get in contact with The iFISH with any further tank stocking questions.

Powered by Top Rated Local®