How Many Fish Can You Settle In An Aquarium?

Every owner of the aquarium sooner or later wonders: how many fish can you keep in the aquarium? After all, an aquarium is not sold with instructions for settling animals in it. As a result, almost all novice aquarists unwittingly overload capacity with fish, often with a catastrophic result. Unfortunately, there is no simple way to determine the number of the future population, but there are a number of factors and methods for calculating the level of a safe number of aquarium fish.

Rule one: 1 cm fish for 1.5 liters of water

This is the most well-known and widespread rule for calculating the number of fish allowed in an aquarium. Of course, this is only a rough estimate. When using this rule, many mistakes are often made: it does not take into account the use of modern filtration systems, the presence of live plants in the aquarium, the sufficiency of lighting and the types of fish. As well as people, fish of even one species often differ in size and shape. Provide 30 liters for 20 cm graceful danios rerio is not the same as the same amount to provide for 20 cm length of a goldfish! Goldfish grow to very large sizes, emit much more waste and need more oxygen dissolved in water, so they have a special calculation of the amount of water they need. And for the most part, larger aquarium fish emit much more waste into the water, and therefore need more water.

You must also take into account the necessary place for swimming, and some fish need more space for this. Despite the fact that the calculations look pretty convincing, in fact, the tank may be too small for the fish to move normally. This is especially true for active and schooling fish. Approximately the required length of the aquarium can be found by multiplying the length of the fish by 8-10. That is, for the maintenance of 5-cm fish you need an aquarium 50 cm long.

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Another caveat: schooling fish need to be kept in a group, which means more space is required. Adding even one or two individuals with too small a volume will result in stress to the fish and shorten their lifespan.

In addition, young fish that have not yet reached adulthood are usually sold on sale. A small charming catfish with a length of 2 cm can reach 30 cm when it grows up, so the calculation should be done on the true size of adult fish, and it is better to foresee the necessary volume in advance before buying an aquarium. Many owners have no idea how many years selected aquarium fish live and how big they grow. So before you buy, study the desired fish in the catalog, find out the actual size of the adult.

Not decent sellers of aquarium living creatures often mislead future breeders, asserting that large fish in a small aquarium will slow down their growth and remain small. This is partly true: in a small volume of fish they slow down in growth, however, their health suffers greatly, and ultimately this leads to a decrease in life expectancy and early death. If a large fish does grow in a small volume, then sooner or later it will lead to excessive regular pollution, constant fish intoxication and all the same sad consequences. Often some ignorant breeders with the enviable persistence get the same favorite type of large aquarium fish in a small aquarium, and do not even suspect that the fish die regularly for one simple reason. an unsuitable aquarium.

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Another error occurs if the owner considers only the net volume of water in his aquarium. However, if a 30-liter aquarium is filled with soil, stones, plants, decorations, equipment, then in fact it does not have 30 liters of water. In fact, the volume of water in an aquarium is usually 10-15% less than the indicated size of the tank.

So it is obvious that although the rule of 1 cm by 1.5 liters of water is quite reasonable, it also has its drawbacks.

Rule Two: Calculate Surface Area

The larger the surface area of ​​the water, the more actively the water is saturated with oxygen, which in turn provides for the needs of more fish. Therefore, the surface area directly affects how many fish you can keep in an aquarium. A tall and narrow aquarium may contain the same amount of water as a low wide one, but they have different surface areas.

The surface area of ​​the water is calculated as follows: the length of the tank is multiplied by the width. In accordance with the water surface area rule, an aquarium should be provided with 12 sq. Cm of surface area per 1 cm of fish.

However, this calculation has the same drawbacks as the previous rule. For example, it is designed only for fairly slim fish, which is not always the case. If the aquarium contains more rounded fish, the calculation should be 20 sq. Cm per 1 cm of fish.

How Many Fish Can You Settle In An Aquarium?

The method of calculating the surface area is not perfect, because many fish need more oxygen, and hence more surface area. So all the same goldfish need water highly saturated with oxygen, as well as fish that live in nature in fast streams and rivers. The advantage of this method is that it takes into account the unusual forms of aquariums.

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What rule to use?

In normal situations, the rule of three liters per 2 cm works quite adequately and is very easily calculated. If you use this rule, always consider the net volume of water and take into account the size of the adult fish, as well as its shape. If the tank is substandard (round or high cube), the surface rule works better than the standard three-liter rule. In any case, always do preliminary calculations, and err on the down side rather than the big one.

Also, never populate the aquarium at once with the entire population. It is recommended to occupy the tank at 25% of the possible amount of fish at a time. Fish waste is toxic, it is processed by colonies of beneficial bacteria, and a bacterial colony needs time to grow and adapt to changes in bio-load. If the fish are settled in stages, the bacteria will have enough time to grow and process the toxins secreted by the fish.


Keep in mind that filtering also greatly affects how many fish you can keep in an aquarium. The filter should allow three to four volumes of water in the tank per hour (five volumes are recommended for goldfish). This means that a 30-liter aquarium needs a filter with a capacity of at least 90 liters per hour. If you are unsure of the choice, it is better to take a more powerful filter, since over-filtering does not pose a danger to the inhabitants of the aquarium.

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