How many fish can I put in my aquarium? Is there a rule of thumb for this? These questions are asked above all by beginners in aquarium keeping who are starting this hobby from scratch. What aquarium owners should look out for when planning the stocking density, we show in this article of our Aquascaping Wiki.

Avoiding overstocking in the aquarium

In principle, basic thoughts on stocking density, and thus the rules of thumb associated with it, aim at species-appropriate care and, above all, at preventing overstocking of the aquarium. Besides the animals' discomfort in social interaction (for example when territories form), overstocking mainly has biological effects: the water is burdened too heavily with pollutants from the many inhabitants. If the owner's maintenance measures and the aquarium's technical setup are not sufficient here, there is a risk that the entire ecosystem in the aquarium will collapse. This situation should therefore be avoided at all costs.

Minimum requirements

The legal recommendation in Germany calls for a glass tank with at least 54 liters and an edge length of 60 cm for keeping aquarium fish. In Austria, it is even prohibited to keep fish in a smaller aquarium. You are therefore on the safe side if you plan at least a tank with these standard dimensions for keeping fish. Smaller tanks are, by the way, called nano aquariums. These are much more suitable for keeping invertebrates such as dwarf shrimp, crayfish or snails. Sometimes, however, keeping fish in such a nano aquarium is possible. We have put together an overview of nano fish that stay small in this article.

Rule of thumb for calculating fish stocking

In the aquarium scene, certain rules of thumb with fish-per-liter or fish-per-centimeter figures are in circulation. Over the years, the following rules of thumb, for example, have become established:

• 1 cm of fish length per 1 liter of aquarium water
• 1 cm of fish length per 30 cm² of area in the aquarium

It should be kept in mind that these rules of thumb only offer a very rough indication and in individual cases are not recommendable at all. As a negative example, it should be mentioned that according to the fish-per-liter formula, two fish of 27 cm each could fit in a standard aquarium with 54 liters of volume. Of course, that is not the case.
With such length figures, it is always important to note that they refer to the expected final size of a fully grown fish. When buying new fish, you often find juveniles in stores, whose size should not be used as a basis for calculation.

Relying solely on these blanket figures therefore does not really make sense in terms of animal welfare. Rather, the stocking density depends on many other factors that have to be taken into account. The aquarist can instead use the results of the rules of thumb as first, very rough guideline values and refine or correct the calculations with the other aspects to be considered.

The Congo tetra is a schooling fish that already grows fairly large and needs a correspondingly large aquarium volume.

Factors to consider when choosing livestock

In fact, the stocking density in an aquarium depends on many more things, which we would like to introduce here in more detail. If you take these aspects into account as an aquarium owner, it is somewhat more complicated, but much more animal-friendly when planning your fish stocking.

• Size of the aquarium: The size of the glass tank does not only mean the pure volume of contents but also the shape, depth, height and edge length of an aquarium. Fish species that love to swim in particular need species-appropriate open swimming space, so a corresponding edge length matters. An aquarium with a lot of depth and decoration items in it offers many hiding places, which are very important for some animal species. For very deep-bodied fish species such as angelfish or discus, certain minimum tank heights are required. For bottom fish, the footprint (that is, length times depth) is rather decisive. As you can see, the requirements for the size of the aquarium can be quite different. When buying fish, therefore, check the respective profiles for what specifications they give regarding aquarium size. In general: larger fish belong in a large aquarium.
• Different water zones: Here the aquarist basically distinguishes between three different swimming zones in which the animals mainly stay. The lower zone is the favorite of many bottom-digging fish with a downturned mouth such as catfish and loaches, but also invertebrates such as shrimp, crayfish and snails. Near the water surface, it is mainly fish species that love current and oxygen that swim, and they often have an upturned mouth. This means that feeding also takes place primarily in this zone. Topminnows and hatchetfish, for example, are fish species that prefer the upper water zone. In between is the middle zone, which mainly represents a transition from bottom to top. Very many fish that love to swim and have a terminal mouth stay primarily in this zone of the so-called open water.

  Armored catfish are usually found near the bottom.

  If you populate your water zones skillfully, you can even accept a somewhat higher stocking density, since the respective animal species are spread across all height zones in the aquarium. On the other hand, overcrowding only one particular water zone should be avoided at all costs (the middle swimming zone is often affected).
• Social behavior: The aquarist should definitely take social interaction into account when choosing livestock. There are fish species that are rather loners or form territories, but also group animals such as schooling fish that only feel comfortable from a certain number of their own species. Especially during the breeding season or when defending a territory, certain fish such as cichlids can become quite aggressive. The remedy here is to choose a sufficiently large aquarium and to offer enough structure with aquatic plants and other decoration to provide enough hiding places.
• Species diversity: Especially in the popular community aquariums, owners often keep many different animal species together. Here you can remember: the more different species are used, the more different needs regarding the care conditions clash. It is therefore simpler and better for animal welfare to keep the species diversity rather manageable and well matched. Here the aquarist should deal well with food, social behavior, group size and the preferred swimming zones.
• Water parameters: Of course, when choosing livestock, the aquarium owner should make sure to combine species that prefer roughly the same water parameters. These include temperature, pH, general hardness (GH) and carbonate hardness (KH). These conditions are determined on the one hand by the technical setup of the aquarium, but above all by the quality of the source water.

Conclusion

The rules of thumb that are common in aquarium keeping are only conditionally suitable for the final calculation of the stocking density, but they can offer a first indication. The other factors such as the size of the aquarium, social behavior, water parameters, species diversity and preferred water zones should definitely be taken into account as well. Inexperienced aquarists in particular can do better by following the usual fish profiles or by seeking expert advice or the advice of experienced aquarists.