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How Many Fish in Aquarium

Calculate how many fish your aquarium can hold based on volume and fish size

liter
cm
๐Ÿ‡ฉ๐Ÿ‡ฐ All calculations are based on Danish rules, rates, and currency (DKK). This calculator is translated for convenience but applies exclusively to Denmark.

How many fish can your aquarium hold?

One of the most common questions among aquarium enthusiasts is how many fish an aquarium can hold. Overstocking is one of the most frequent mistakes and can lead to poor water quality, disease and stress in fish.

Rule of thumb: 1 litre per cm of fish

The most used rule is 1 litre of water per centimetre of fish (measured at adult size). So a 100-litre aquarium can theoretically hold fish with a total length of 100 cm โ€“ for example 20 fish at 5 cm each. This rule gives a good starting point but should always be adjusted for conditions.

Conservative recommendation

We recommend using 70% of maximum stocking as the recommended number. This provides a safety margin accounting for territorial needs, waste output and filter capacity. Fewer fish means better water quality and healthier, happier fish.

Factors that affect the number

Fish species plays a big role โ€“ territorial species like cichlids need more space than schooling fish like neon tetras. Filter capacity, plant quantity and water change frequency also affect stocking levels. Bottom dwellers do not require the same water volume as upper-level fish.

Always use adult fish size

Always calculate based on the fish's full adult size, not the size at purchase. Many aquarium fish are sold as juveniles and can grow to double or triple their purchase size.

Frequently Asked Questions

Is the rule of 1 litre per cm of fish always correct?

No, the rule is a simplified guideline. It does not account for body shape (a 5 cm discus is much more massive than a 5 cm neon tetra), territorial needs, activity level or waste production. Cichlids and goldfish produce far more waste than neon tetras and therefore need more space per cm.

What happens with overstocking?

Overstocking leads to faster accumulation of ammonia and nitrite, which are toxic to fish. It results in more frequent disease, stress, aggression, poor growth and shortened lifespan. The filter cannot keep up with waste production and water quality deteriorates quickly. In worst cases the entire stock can die.

Do cleanup crew fish count in the calculation?

Yes, all fish count in the calculation โ€“ including bottom-dwelling cleanup fish like Corydoras and Otocinclus. Although they live in a different zone, they still produce waste and load the filter. Many cleanup fish like Plecostomus can also grow very large and require significant space.

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