To know the correct answer to this question is no less important than to have information about the care and the creation of optimal conditions for your underwater pets. On the one hand, everyone would like to fill their aquarium with various fish as much as possible; on the other hand, overpopulation cannot be allowed, which is fraught with not only limiting the scope for swimming, but also diseases and death of inhabitants. In this article we will try to figure out how to find a reasonable compromise and not “overload” your aquarium.

## How not to do

It is often found advice to choose the number of fish, starting from the volume or number of liters of tank in which they will live. The following is usually taken as an axiom: for one fish, the optimal amount is 5 liters of water. Accordingly, in a 100-liter aquarium, you can put 20 fish, etc.

Or even more interesting: 1 liter (or gallon) of water should be 1 cm (or inch) the length of the fish. One of the professional aquarists was conducted speculative experiment, which completely refuted the correctness of this formula. He mentally picked up a 100-liter classic aquarium and began to try on different fish.

- First, neon (3 cm). it turned out that 33 pieces will be too small, they will have to look for in such a volume.

- Then there was a golden fish (10 cm). From ten individuals of this breed there will be so much waste, that it is necessary to “rake them with a shovel”, and even powerful filters will have a hard time dealing with this problem.

- Further there was an astronotus (25 cm). According to the formula, we can contain four giants in 100 liters, but in real life it is unrealistic to grow even two in such conditions.

- And finally there was a protopterus (1 m). You can, of course, try to “shove” it into a vessel of a given volume, but will such an existence be comfortable? Unlikely.

And from them, unfortunately, you do not know that schooling fish can be settled in more compact groups, that “loners” need space, and fighting breeds are better kept separate. They do not say anything about repackaging a particular layer of water, and each fish prefers its own layer of water (upper, middle or lower). And about much more. What do you need to consider to make the right calculations?

## Factors determining the number of fish in an aquarium

- Let’s start with the physico-chemical parameters of water (acidity, the amount of nitrogen compounds, O2 saturation and temperature).

The most important limiting factor is the content of oxygen in the water. Plants and technical means (as additional sources of this gas) make it possible to increase the density of fish landing.

To determine its limit is simple: if the fish try to spend most of the time near the surface, convulsively swallowing the air (with high-quality aeration!), Then you have overpopulation.

At the same time, it is also necessary to check the temperature of the water, since when it rises, oxygen dissolves worse in water.

- The next factor is the number, size, weight, age, food activity and growth rate of the fish.

The larger the individual, the more discharge from it. They, as well as food residues, decompose and release toxic ammonia. If there are too many fish, then he simply will not have time to recycle and will poison the aquarium. The permissible number of fish can be identified by conducting chemical tests of water for nitrogen, ammonia, ammonium, nitrite, nitrate and good results by analyzing the results.

- It is important whether there is soil in the aquarium, what quality it is and in what quantity.
- Do live plants and what kind they are.
- Is filtering equipment used and which; how often the aquarium is serviced (water is replaced, the ground is cleaned, etc.).
- How often and in what quantities the food is given, what quality and chemical composition it has, what percentage of fish they eat. There are other, but not so important factors.

## Methods for optimal filling of the aquarium

In aquarism there are several options for determining the optimal filling of the reservoir. The simplest, but also the most controversial is called the rule "3.5 cm by 5 liters of water", where 3.5 cm is the total length of all the fish. Here are your pitfalls:

- You need to know in advance the final size of an adult.
- It is necessary to take into account the size and shape of fish. Large full inhabitants produce much more waste, which means they need more water than they are elongated and slender.
- You need to take into account only the actual amount of water, minus occupied by soil, plants and other accessories, which is a minus about 15 percent.

The basis of the following method is the determination of the surface area of the water. (To find it, you need to multiply the width and length of the aquarium.) The larger it is, the more intense the oxygen exchange and the greater the number of fish that can be planted in a tank. For large fish, a ratio of 3 cm body to 150 cm² of surface is considered optimal, for elongated ones. to 90 cm².

## Some more important requirements

- Use the rule "1 cm by 1 l" only for small, thin, non-aggressive and unpretentious fish.
- The greater the length and height of the body of the fish, the greater the volume of water she needs. For example, a 20-centimeter discus requires at least 40 liters of water.
- Thick and slimy fish need more water. For example, a 25 cm goldfish needs at least 30 liters.
- Small aquariums are only suitable for small fish.
- The more aggressive the fish, the more volume it needs.
- Schooling fish is best kept in large aquariums.
- In a densely planted aquarium with good filtration can live more fish.
- Select the inhabitants should be so that evenly populate all layers. One can find out who is who by a simplified scheme: the mouth is turned upwards — the upper layer is directed along the middle line — the middle layer, looking down — the bottom rock.

## Specific fish. specific liters

Some practicing authors share their experiences and tell which species, in what quantities and aquariums they feel great (with the condition of normal filtration and regular maintenance). Here are some such tips:

- Small fish (up to 4 cm: neon, cardinal, rasbor, guppy) can be kept in aquariums from 10 liters with a density of landing of 1 l per 1 fish.
- Small (up to 6 cm: pecillia, ternation, hasemania, rhodostomus, minor, barbus, guppy) in a 20-liter planting density of 1.5 liters per 1 fish.
- Small peaceful ones (up to 10 cm: swordtail, mollies, congo barbs, cross and black apistogram) in aquariums not less than 150 l. Density 3-10 liters per 1 fish. And for schooling water you need less, and for singles. from 5 liters per piece.
- Medium peaceful (up to 20 cm: angelfish, golden, gourami, danio malabarian) in an aquarium from 200 l. The rules are hard to call, many exceptions. It all depends on the fish, its mass, behavior, habits. The greater the volume, the more you can increase the density of landing.
- Small cichlids up to 10 cm need 40 liters for a pair.
- Malawian cichlids: an aquarium of 150 liters, where 1 fish accounts for no more than 10 liters. Such overpopulation is even necessary, it reduces their aggression.
- Large fish (up to 30 cm: akara, astronotus, cichlazoma) are placed in a pair of 250 liters or ten in 500 liters.
- Discus fish need an aquarium of 200 liters and at least 50 liters for 1 fish.
- Very large ones (arvana, snakeheads, clarium catfish) are best settled in aquariums at least 1.5 m in length. For 500 l 1-2 fish.
- Soma and fights. They are bottom, they can not be taken into account in the total. The larger the aquarium, the larger the breed can be planted. "Sucker" one, "digging" no more than 5.
- Labyrinth. One Betta Fish will be enough 0.5-2 liters. Gourami. 20 liters per pair.

What can be drawn from all this? And such that the recommendations need to listen, but not blindly follow the rules, the more outdated. Each aquarium is unique and an individual approach is needed. We hope that our information will help you to correctly solve the problem “how many fish can you keep in an aquarium”.