You need to know how many fish to put in your aquarium, taking into account its size. Length, height and liter are important factors when choosing species and how many specimens we can have of each of these species. But then, how to calculate **how many fish we can have in an aquarium?**

There are some rules used to calculate the amount of fish suitable for an aquarium. For example: centimeter rule, size rule and maximum length rule. The first considers just how many centimeters the fish is. The second considers how many centimeters, **but also the size of the fish**. The third rule considers that the maximum length of the fish must also be defined according to the total size of the aquarium. We will discuss the reasons for doing these calculations and how to calculate each of these rules.

### Why limit the number of fish in your aquarium?

When we are starting in the hobby, the desire is to put the largest number of fish and populate our aquarium with the most different species. This explains why many starter tanks have a high mortality rate, cloudy and foul-smelling water. Overpopulation in the aquarium considerably increases the amount of feed needed in the tank and obviously this also increases the amount of waste in the water.

Both factors generate intense growth of ammonia and nitrite. Depending on the amount of fish in your aquarium, the concentration of pollutants is so high that your filter cannot break down such a large amount of harmful substances, generating high mortality in your fish, dirty water, bad smell, uncontrolled spread of algae, etc. .

Due to this clear need to control the number of inhabitants in your aquarium, methods have emerged over time to calculate the ideal amount of fish. It must be understood that there is no perfect way to calculate this number. Aquariums are living ecosystems, and each one reacts differently to the same conditions. Likewise, each animal is a unique organism, so it is not possible to perfectly generalize how much pollution a fish of each species generates.

Even so, rules were created that help the aquarist when deciding how many fish to put in the aquarium. One of the first to emerge was the centimeter rule, which is now considered outdated. The most indicated method at the moment is to use the size rule in conjunction with the maximum length rule.

It is also worth remembering that these calculations are to calculate the total amount of fish in your aquarium, but each species also has an indication of minimum aquarium size, according to its behavior and size.

### The centimeter rule (outdated)

One of the first to appear, it was already a great advance when it came to calculating the ideal amount of fish for the aquarium. The rule considers for this calculation only the length of the fish, defining a specific litre for each fish. It is important to remember, that the size of the adult fish is considered , not the actual size of the animal.

The rule is very simple and says the following: For every 1 cm of fish you need to have 1 liter of water.

That is, a 4 cm fish, a thick bush for example, would need 4 liters of water. So if you have a shoal with 10 thick bushes, you would need a 40 liter aquarium to properly accommodate your fauna.

In some cases, this rule works perfectly, especially with small fish. But when we start thinking about large fish, the rule’s effectiveness seems to run out. Let’s think about a specific case.

If you want to create an aquarium for an Oscar , you need to understand the liter for a fish of this size. The adult Oscar can be up to 40 cm, in addition to being a very active and even aggressive fish. If we use the centimeter rule, what would this calculation look like?

1 Oscar = 40 cm. So we would need an aquarium of only 40 liters. If you’ve ever bred this fish, or even seen one, it’s pretty clear that the centimeter rule doesn’t work at all for this animal. And the same is observed with most large fish. That’s why the size rule came about.

### The Gate Rule

As aquarists used the centimeter rule, it was noticed that the larger the size of the fish, the more liters per centimeter it needed. In this way, adaptations to the centimeter rule emerged, now taking into account the different sizes of the fish.

The size rule, although a little more complicated, is a much more efficient and interesting approach when calculating the number of fish for your aquarium. It works as follows:

**Fish from 2 to 5 cm must have 1.5 liters per cm available**

**Fish from 6 to 9 cm must have 2 liters per cm available**

**Fish from 10 to 13 cm must have 3 liters per cm available**

**Fish of 14 cm or more must have available 4 liters per cm**

Now, if we recalculate the Oscar, we will arrive at a value of 160 liters, which is much more appropriate for an animal of this size. This rule usually generates slightly higher requirements than the centimeter rule, but they are much more appropriate values for healthy, stress-free fish.

### The maximum length rule

This rule isn’t used as much as the previous ones, but it’s also a great guide. It arose due to the fish’s need to move easily in an aquarium.

**For example, if you have a 40 cm fish in your tank, the size rule would tell you that you need a 160 liter tank, but a 160 liter tank is probably only 90 to 100 cm in front and 40 cm in height. background. These dimensions give the animal very little room to maneuver. To guarantee a little more space for the fish to move, the maximum length rule defines:**

The maximum length of the fish should be 10% to 15% of the total aquarium volume. In other words, an aquarium of 160 liters should hold a maximum of fish up to 22 cm.

### Examples of calculations:

To be very clear how the calculations are done, let’s consider some typical aquarium sizes and size the fauna according to the liter. Always remembering to respect the maximum length rule. Examples below:

#### How many fish to put in a 36 liter aquarium:

**Small fish:** 36 (literage) / 1.5 (liters/cm for small fish) = 24 cm in total for small fish. So you can have, for example, 8 3 cm fish in that aquarium.

In such a small aquarium, we can only have small fish.

#### How many fish to put in a 40 liter aquarium:

**Small fish:** 40 (literage) / 1.5 (liters/cm for small fish) = 27 cm in total for small fish. So you can have, for example, 9 3 cm fish in that aquarium.

In such a small aquarium, we can only have small fish.

#### How many fish to put in a 50 liter aquarium:

**Small fish:** 50 (literage) / 1.5 (liters/cm for small fish) = 33 cm in total for small fish. So you can have, for example, 11 3 cm fish in that aquarium.

**Medium fish:** 50 (literage) / 2 (liters/cm for medium fish) = 25 cm in total for medium fish. So you can have, for example, 4 to 5 6 cm fish in that tank.

In an aquarium of this size, we cannot have big fish.

#### How many fish to put in an 80 liter aquarium:

**Small fish:** 80 (literage) / 1.5 (liters/cm for small fish) = 53 cm in total for small fish. So you can have, for example, 17 to 18 3 cm fish in that tank.

**Medium fish:** 80 (literage) / 2 (liters/cm for medium fish) = 40 cm in total for medium fish. So you can have, for example, 6 to 7 6 cm fish in that tank.

**Large fish:** 80 (literage) / 3 (liters/cm for large fish) = 26 cm in total for large fish. So you can have, for example, 2 to 3 10 cm fish in that aquarium.

In an aquarium of this size, we cannot have very large fish.

#### How many fish to put in a 100 liter aquarium:

**Small fish:** 100 (literage) / 1.5 (liters/cm for small fish) = 66 cm in total for small fish. So you can have, for example, 22 3 cm fish in that aquarium.

**Medium fish:** 100 (literage) / 2 (liters/cm for medium fish) = 50 cm in total for medium fish. So you can have, for example, 8 to 9 6 cm fish in that tank.

**Large fish:** 100 (literage) / 3 (liters/cm for large fish) = 33 cm in total for large fish. So you can have, for example, 3 to 4 10 cm fish in that aquarium.

In an aquarium of this size, we cannot have very large fish.

#### How many fish to put in a 200 liter aquarium:

**Small fish:** 200 (literage) / 1.5 (liters/cm for small fish) = 133 cm in total for small fish. So you can have, for example, 44 3 cm fish in that aquarium.

**Medium fish:** 200 (literage) / 2 (liters/cm for medium fish) = 100 cm in total for medium fish. So you can have, for example, 16 to 17 6 cm fish in that tank.

**Large fish:** 200 (literage) / 3 (liters/cm for large fish) = 66 cm in total for large fish. So you can have, for example, 6 to 7 10 cm fish in that aquarium.

**Very large fish:** 200 (liters) / 4 (liters/cm for large fish) = 50 cm in total for very large fish. So you can have, for example, 3 to 4 14 cm fish in that tank.

These rules and values are a guide, but not foolproof. So adapt your reality. In any case, if you decide to put in more fish than recommended by the rules, don’t overdo it and compensate for this excess with an even more powerful filter than usual, so that it can handle the extra load generated by the fish.

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