Start with the aquarium's actual water volume, then consider stocking, fish waste, flow tolerance and the filter's media capacity. Manufacturer tank-size ratings are only a starting point. A lightly stocked planted tank may need modest flow, while a heavily stocked cichlid or goldfish tank benefits from more filtration. Use the filter-flow calculator to compare claimed flow with the turnover you want, then check that the filter can hold coarse mechanical and biological media.
Two filters both labelled 'for 200 litres' can perform very differently because one may offer far more usable media volume or maintain flow better as it gets dirty. Size the whole filtration system, not just the pump number.
Start with actual water volume
Tank labels describe gross volume, but substrate, decor and an unfilled rim reduce the water actually present. Use actual volume when calculating turnover so the result is realistic.
Our tank-volume and filter-flow tools can help with both figures.
Turnover is a guide, not a law
Many freshwater setups work well with several tank volumes per hour, but there is no universal number for every fish. Bettas and long-finned species prefer gentler flow than riverine fish, while messy fish may need stronger mechanical filtration.
Judge the fish's behaviour and the amount of waste as well as the maths.
Media capacity matters
- Coarse sponge catches debris without clogging too quickly.
- Biological media provides well-oxygenated surface area for nitrifying bacteria.
- Fine floss is useful for polishing but should not be the only mechanical stage.
- Chemical media is optional for specific jobs rather than a substitute for biological filtration.
Allow for dirty-filter flow loss
Headline flow is normally measured under favourable conditions. Real flow falls with height, hoses, elbows and clogged media.
A little capacity margin is sensible, provided the outlet can be adjusted or baffled if the fish dislike strong current.
When two smaller filters make sense
Large or heavily stocked tanks may benefit from two filters for better circulation and redundancy. That is different from deliberately creating excessive current.
Split the outlets so water moves through the tank without turning it into a washing machine.
Frequently asked questions
Should I buy a filter rated for a bigger tank?
Often a modest step up is useful because media capacity and real-world flow decline as the filter gets dirty, but avoid creating unsuitable current.
Is litres per hour the most important number?
No. Flow matters, but so do media capacity, maintenance access and the needs of the fish.
Can a filter be too big?
The media capacity is rarely the problem; excessive current, intake risk or impractical maintenance can be.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.