Pump output, circulation or maximum flow?
Pump output rates the pump itself, and how it was measured is the maker’s choice. Filter circulation is the maker’s figure for water passing through the filter, and it is lower: Fluval measured the 07 Series circulation with intake and output hoses of the same length. Some makers give a maximum flow instead, which holds only under the conditions they state with it. A shop listing that just says “GPH” can mean any of these, and none of them is the flow you would measure in your own tank with your hoses and media.
Our comparison keeps all three in separate rows. Where a maker gives no circulation figure, the row says “Not published” rather than borrowing the bigger number.
A real example: Fluval 107 and 307
Fluval lists the 107 with 145 GPH pump output and 95 GPH filter circulation. For the 307, the figures are 303 and 206 GPH. Selecting the larger number would make either filter appear to move more water through its filter system than the maker’s own circulation figure.
These are published ratings, not measurements taken in your aquarium. See the 107 page and 307 page for each model and its manual.
Why your filter moves less water in your tank
Ratings are measured on the maker’s bench. In your tank the water also has to climb from the cabinet to the rim, push through media that fills with dirt, and run through hoses and fittings, so the flow you get is lower and drops further between cleanings. Lift is the big one for a canister: Fluval gives the 107 a maximum head of 4.75 ft, the height at which its pump can no longer push water up at all.
To know what you actually have, measure it: catch the outflow in a jug for a timed spell, and the filter flow calculator turns that into gallons per hour and how many times an hour it moves your tank’s water. Treat that turnover as a guide, not a target every tank must hit: bettas, long-finned fish and dense plants want gentle water, while fish from rivers like a current. If the flow falls soon after a clean, check the intake sponge, the hoses for kinks and the impeller.
How to compare two filters
- Start with the type that fits your space: a hang-on-back needs rim and rear clearance, a canister needs cabinet room and a route for its hoses. Filter types
- Check the maker rates it for your tank size.
- Compare flow on the same kind of number: circulation with circulation, maximum with maximum. Fluval lists the FX4 at 700 GPH from the pump and 450 GPH through the filter; AquaClear and Seachem give a maximum flow only.
- Check you can lift the media out without moving the tank, and read the manual’s priming and flow-control steps.
If one maker gives only pump output and the other only circulation, the two numbers can’t tell you which moves more water once installed: measure both, or choose on fit and upkeep. Every filter on this site keeps the maker’s numbers in separate rows: all filters · Fluval’s sizes side by side