Good circulation keeps oxygen, heat, and nutrients evenly distributed and stops detritus settling. This calculator turns a target turnover rate into the concrete flow figure your filters and powerheads must deliver, and shows the right turnover band for your type of tank.
How it works
Required flow is simply volume multiplied by turnover:
flow per hour = tank volume × turnover (times per hour)So a 200 litre reef tank at 25x turnover needs 5000 litres per hour of combined circulation. Because real pumps lose flow to media and head height, the tool also suggests a purchase rating about 35 percent higher than the in-tank target.
Recommended turnover by tank type
| Tank type | Turnover target | Notes |
|---|---|---|
| Planted (low-tech) | 4–6x | Gentle flow; avoid uprooting stem plants |
| Planted (high-tech CO₂) | 5–8x | Distribute CO₂ without blowing it off the surface |
| Fish-only (community) | 5–8x | Balanced for most tropical species |
| Cichlid / African rift lake | 8–12x | Cichlids are active and produce high waste loads |
| Reef (LPS / soft coral) | 15–25x | Moderate flow; avoid direct blasts on open LPS |
| Reef (SPS, frag tank) | 25–40x | High, turbulent, randomised flow |
Worked example
A 300 litre mixed reef with LPS corals targeted at 20x turnover needs 300 × 20 = 6000 LPH of total circulation. With a 35% head-height derating, the pump or pumps should be rated for about 6000 × 1.35 = 8100 LPH. A return pump supplying 3500 LPH and two powerheads at 2500 LPH each delivers 8500 LPH — just above the purchase-rating target and fine in practice.
Practical tips
Combine flow sources. In a reef, split the total across a return pump and one or more powerheads or wavemakers. A single pump producing a laminar jet will create dead spots elsewhere in the tank.
Eliminate dead spots first. Detritus accumulates where flow falls below roughly 2–3x. Angle a powerhead toward corners and under overhangs where mulm builds up, then tune the total flow for the livestock.
Account for head height. Every metre of vertical lift and each 90° elbow in the return line cuts pump output. A rated 3000 LPH pump at 1.2 m of head might deliver only 2000 LPH in the tank — always read the pump’s head-loss curve before sizing.
Dial back for delicate inhabitants. Bettas, fancy goldfish, seahorses, and small shrimp are stressed by strong currents. For those setups, stay at the low end of the fish-only range and use a sponge filter or a spray bar to diffuse the outlet flow.
Seasonal adjustments. Planted tanks often benefit from a slight flow increase during the lights-on period to distribute CO₂ and a reduction overnight when gas injection is off, to let CO₂ levels recover.