Filter shopping is confusing because manufacturers publish flow in different conditions. Some ratings are measured with an empty filter, no media and no lift. Once the filter contains sponges, biological media and tubing, the delivered flow can drop. Choose the filter by turnover and biological capacity together.
Turnover rate in one calculation
Turnover rate is the filter flow divided by the aquarium volume. For a 20-gallon aquarium, a 200 GPH nominal filter represents 10 nominal turnovers per hour. In metric units, 1 US gallon is 3.785 litres and 1 GPH is about 3.785 litres per hour.
| Tank volume | Gentle starting range | General community range | High bioload or reef starting range |
|---|---|---|---|
| 20 gal / 75 L | 300–450 L/h | 450–750 L/h | 750–1,000 L/h, diffused if needed |
| 30 gal / 115 L | 450–700 L/h | 700–1,150 L/h | 1,150–1,500 L/h, with suitable flow |
| 55 gal / 208 L | 800–1,200 L/h | 1,200–2,000 L/h | 2,000 L/h or multiple filters |
These ranges are planning points, not universal rules. A heavily planted low-stock tank may need less biological capacity than a bare tank with messy fish. A reef or high-oxygen setup may need more circulation, while a betta aquarium may need a gentler outlet.
Rated flow versus useful flow
- Head height: lifting water from the cabinet reduces pump output.
- Media resistance: fine mechanical pads clog and restrict flow before they look dirty.
- Pipe and intake design: narrow tubing, bends and blocked intakes add resistance.
- Outlet distribution: a wide spray bar can deliver the same flow more gently than a narrow jet.
- Maintenance: impellers, sponges and hoses need cleaning to keep flow near the design level.
How to choose flow for your fish
- Start with usable water volume, not the aquarium's outside dimensions.
- Choose a nominal rating that leaves room for the drop caused by media and lift.
- Check whether the species prefer calm, moderate or fast-moving water.
- Provide enough biological media and cycle the filter before adding a full stock.
- Use water tests, fish behaviour and nitrate trends to confirm the setup.
Check filter capacity with your stocking plan
Aquapacity combines flow, biological efficiency, water changes and species bioload in one result.
Calculate filter capacityOne filter or two?
Two smaller filters can give better circulation and useful redundancy: one can keep running while the other is cleaned. They also let you direct flow to different parts of the aquarium. The trade-off is more equipment and two maintenance routines. Whatever the arrangement, count the biological media and the actual maintenance habit, not just the sum of pump labels.
Frequently asked questions
Is 10 times turnover always better?
No. More flow is only better when fish tolerate it and the filter has enough biological media. High flow that exhausts fish or creates dead spots around decorations is not an upgrade.
How often should filter media be cleaned?
Clean mechanical media when flow drops or it is visibly blocked, using removed aquarium water where possible. Avoid replacing all biological media at once because that can remove much of the established bacterial colony.
Should I use the manufacturer's maximum GPH?
Use it as a comparison point, not as a guaranteed result. Look for the stated test conditions and plan for lower flow once the filter is installed and loaded.