Is the “One Inch of Fish per One Gallon of Water” Rule: the Gold Standard for Fish Stocking Levels?

For most aquarists, their aquarium is a work in progress, a work of art that is the result of months or years of fishkeeping decisions to create a living work of art for their home environments. Whether you are a saltwater or freshwater aficionado, LiveAquaria® is here to help you. One of the questions that will arise is how many fish can be kept in an aquarium. In reality, there are a host of other factors to consider when stocking your aquarium.

An estimated fish stocking level rule of thumb that is popular in the aquarium industry is to plan to allow one gallon of water for every one inch of fish. While this may be a starting point for freshwater fish, calculating what size of aquarium to use for marine fish is more complicated. Coupled with this rule, many people suggest that fifteen percent of the aquarium’s capacity should be subtracted right off the top to account for plants, equipment, and aquascaping present in the aquarium, before calculating fish stocking levels using this rule. We’ll discuss other factors that come into play with this suggested stocking level standard.

Keep in mind, when calculating fish size, it is important to make these calculations on the adult size of the fish, which can vary from the size at the time of purchase. Care Statistics such as those posted on aquatic life description pages on LiveAquaria® are informative ways to research adult sizes. The height of the fish should also be taken into account, to ensure there is adequate area for swimming. Consideration should also be given to the shape of the fish, as full-bodied fish can create a larger demand on filtration systems.

There are other ways to calculate fish stocking levels other than the “one inch for one gallon” estimate. Let’s discuss two of them here.

A second accepted way to compute stocking levels is to take the adult length size of a fish and multiply it by six to determine the length of the aquarium needed. Then the depth and height should be twice the adult length of the fish. Using this method, a twelve inch fish would require an aquarium that is at least 72”x24”x24” in size.

A third way to compute aquarium stocking levels is by the Surface Area rule. This advocates ensuring that there are twelve square inches for each one inch of fish. Computing with this strategy, a fish twelve inches in length would require 144 square inches of area. Is this method one hundred percent accurate? No, because it doesn’t take into account whether fish are wide bodied or slender bodied. The dimensions of an aquarium are important because the surface area will differ depending on the dimensions. For example, a tall and thin aquarium will have a vastly different surface area than a longer, shorter tank. The surface area will be a good indicator of the amount of dissolved oxygen that will be present in the water. The greater the surface area, the greater number of fish (or bigger fish) can be housed.

Regardless of the fish stocking estimates used, aquarists should resist the urge to bypass general guidelines with the desire for more fish. This is necessary to provide all inhabitants as stress free a life as possible, and to keep them healthy and safe. Overstocking should be avoided, as it can lead to aggressive and territorial behavior, as well as result in poor water quality and ammonia spikes.

In determining adequate aquarium sizes, aquarists must be sure to provide enough space so every inhabitant can establish territories. When aquarium water capacity is stated, the dimensions of the aquarium also come into play, as well as the species to be housed, the number of fish to be housed, fish behavior, and filtration demands, to name a few.

In addition to accepted fish stocking level guidelines, Part 2 in this series will discuss how aquarium requirements, filtration, and fish behavior factor into stocking levels.