Sizing a Filter to Your Tank, Not to a Flow Rate

Flow rate is the number printed largest on the box and the least useful one in isolation.

Start with the maker rating

Filters are rated for a tank volume. That rating already accounts for the media volume and the expected bioload — it is a better starting point than litres per hour.

Leave headroom

A filter rated to exactly your tank size will be working at its limit from day one. Sizing one step up costs little and gives you room for a heavier stock later.

Where flow does matter

Bettas and other still-water species dislike strong current. In those tanks a lower flow, or a filter with an adjustable outlet, is the right answer.