Tank Stocking: The Truth About The 1 Inch Per Gallon Rule

Kate Barrington
by Kate Barrington
You’ve heard about the “one inch of fish per gallon” rule, but there are other factors to keep in mind when you’re first stocking your aquarium.

For new aquarium hobbyists, the excitement of starting a new tank can be powerful. If you really want to make sure that you start your tank off right, however, you need to take your time not only in selecting the type of fish you want to keep, but the number as well. Many novice aquarium hobbyists latch on to the “one inch of fish per gallon” rule but this concept is meant to be more of a guideline than a hard and fast rule.

What is the “One Inch Per Gallon” Rule?

The “one inch of fish per gallon” rule was designed to help novice aquarium hobbyists avoid overstocking their tanks. When you start a new aquarium it takes time for the nitrogen cycle to become established and if you put too many fish in your tank at once, you could overload the tank and you could end up with a bacterial bloom that might kill your fish. To prevent this from happening, new aquarium hobbyists are encouraged to stock their tanks with no more than one inch of fish per gallon of tank capacity. For example, if you had a 20-gallon tank you would be advised not to stock it with fish that add up to a total of more than 20 inches in length. This rule is great to use for a guideline, but there are many other factors to consider when stocking your tank.

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Other Factors to Consider

What many novice aquarium hobbyists fail to realize about the “one inch of fish per gallon” rule is that it applies to the maximum length of the fish, not their current length. Most pet stores sell juvenile fish, so you have to keep in mind that the fish you bring home from the pet store are going to grow. That juvenile discus fish you pick out at the pet store may only measure 2 inches in length right now, but it will grow another six to eight inches by the time it is full-size. So, when using the one inch per gallon rule, make sure you factor in the maximum size of your fish.

Another factor you need to think about when stocking your tank is that not all fish have the same body shapes. If you look at a discus fish, for example, you will notice that it has a laterally compressed body – this is true for most cichlid species. If you look at a catfish, however, you will notice a much fuller body shape. Fish with laterally compressed bodies take up less space in the tank than wider species, even if they are the same length.

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You also have to think about the amount of waste the fish in your tank are going to produce. The more waste your fish produce, the greater the “biological load” of your tank will be and the more beneficial bacteria you’ll need to break down all of that waste. Smaller fish like danios and tetras, even most angelfish, produce a low to moderate amount of waste. Other fish like goldfish and plecos eat a lot and therefore produce high amounts of waste. If you want to stock your tank with low waste-producing species like tetras, the one inch per gallon rule is a good place to start. For high waste-producing fish, however, your tank won’t be able to handle as many fish. No matter what kind of fish you choose to stock your tank, you need to make sure to maintain high water quality by installing a filtration system and by keeping up with weekly water changes.

The more careful you are about stocking your tank, the more likely you are to be successful. You need to realize that fish are living creatures and different species have different needs. Unless you familiarize yourself with the needs of your fish and take the factors discussed above into account, you might end up dealing with some big problems.

Kate Barrington
Kate Barrington

Kate Barrington is the loving owner of two cats (Bagel and Munchkin) and a noisy herd of guinea pigs. Having grown up with golden retrievers, Kate has a great deal of experience with dogs but labels herself a lover of all pets. Having received a Bachelor's degree in English, Kate has combined her love for pets and her passion for writing to create her own freelance writing business, specializing in the pet niche.

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