A filter that is too weak leaves a tank cloudy and unhealthy, while a filter that is far too strong turns it into a washing machine that exhausts the fish. The right size sits between those extremes, and it depends on tank volume and stocking. This free filter flow calculator gives you a target flow in litres per hour and a sensible shopping range, so you can compare filter boxes with confidence.
Flow is measured as turnover: how many times the filter moves the whole tank volume each hour. A lightly stocked tank does well at about four turnovers an hour, a normal community at six, a heavily stocked tank at eight, and tanks with messy large fish at ten. The same one-hundred-litre tank therefore needs very different filters depending on what lives in it.
Using the calculator is quick. Enter your tank volume in litres and pick your stocking level. Press Calculate filter flow and you get the turnover rate, the target flow in litres per hour, and a shopping range ten percent either side of that target. Filter makers often quote flow with empty media, so real flow in your tank will be a little lower than the box number.
A worked example helps. A one-hundred-litre normal community tank at six turnovers needs six hundred litres per hour, with a shopping range of about five hundred forty to six hundred sixty. A heavily stocked tank of the same size needs eight hundred litres per hour. Buying a filter rated at three hundred for either tank would leave waste building up between changes.
Flow is not the same as filtration quality. A large filter with plenty of sponge and ceramic media holds more helpful bacteria than a small fast filter, even at the same flow rate. When choosing between two filters near your target, pick the one with more media space and adjustable flow, and check that spare sponges are easy to buy for that model.
Strong flow needs managing for slow or long-finned fish. Bettas, fancy goldfish, and many small tetras struggle in a jet of water, so point the outlet at the glass, add a spray bar, or baffle the flow with a sponge on the outlet. Watch the fish after fitting a new filter: fish pinned to one side, or hiding constantly, are telling you the flow is too strong.
Two smaller filters often beat one large one in bigger tanks, since they spread flow, give backup if one fails, and let you clean one at a time without losing all your bacteria. Clean filter sponges in old tank water, never in tap water, and stagger cleaning so the bacteria colony is never wiped out in a single session.
This calculator gives a target flow for planning, not a brand recommendation. Check the real flow with media fitted, and judge the result by water clarity, test readings, and fish behaviour over the first weeks. It is free, it needs no sign-up, and it runs entirely in your browser.
Filter maintenance is where most flow problems actually start. A filter that was perfect on day one slowly loses flow as sponges clog with waste, and owners often blame the fish or the water when the real cause is a dirty impeller. Check flow monthly: hold your hand near the outlet and compare with how it felt when new. Clean the impeller and intake strainer every few months, rinse sponges in old tank water, and never run a filter dry, since that burns out the motor in minutes. Keep a spare impeller if your model sells one, because a broken impeller on a Sunday leaves a tank unfiltered until shops open. A filter that is cleaned on a routine keeps its rated flow for years, while a neglected filter can lose half its flow in a few months without anyone noticing until the water turns cloudy.