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Filter Capacity Calculator

Calculate the recommended filter capacity for your aquarium based on volume, fish count and plants

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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.

Calculate the right filter capacity

A correctly sized filter is crucial for water quality in your aquarium. The filter converts toxic ammonia to nitrite and then to much less harmful nitrate through biological filtration. This calculator helps you find the right filter flow.

Rule of thumb: 3x volume per hour

The standard recommendation is that the filter should pump at least 3 times the aquarium volume per hour. For a 100-litre aquarium this means a filter with flow of at least 300 litres/hour. This ensures sufficient water circulation and biological filtration.

More fish require more filtration

When fish numbers exceed 1 per 10 litres, you should increase filter capacity by 50%. More fish produce more waste, requiring more biological filtration capacity. Overloading the filter leads to ammonia spikes and fish disease.

Positive effect of plants

Many live plants reduce the need for mechanical filtration by about 20%. Plants absorb ammonia and nitrate directly and produce oxygen. A densely planted aquarium can therefore manage with a slightly smaller filter. However, good water circulation is still important for plants.

Choose the right filter

External canister filters are typically most effective for aquariums over 60 litres. Internal filters are good for small aquariums. Hang-on-back filters (HOB) are a good middle ground. Always choose a filter with a bit of extra capacity โ€“ it is better to over-filter than under-filter.

Frequently Asked Questions

Can a filter be too large?

Generally no โ€“ a larger filter does not harm as long as the flow is not too strong for the fish. Betta fish and other long-finned fish struggle with strong current. You can reduce flow with a spray bar or by directing the outlet towards the glass. Over-filtering gives better water quality and more stable biology.

What is biological filtration?

Biological filtration is the process where beneficial bacteria (nitrifying bacteria) convert toxic ammonia from fish waste to nitrite and then to nitrate. These bacteria live primarily in filter media. It typically takes 4-6 weeks to build a sufficient bacterial colony in a new filter โ€“ this is called 'cycling' the aquarium.

Should I turn off the filter at night?

No, the filter should run 24/7. If the filter is turned off for more than a couple of hours, the beneficial bacteria in the filter media begin to die from lack of oxygen. This can destroy the biological filtration and lead to dangerous ammonia spikes when restarted. The filter should only be turned off for maintenance and for as short a time as possible.

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