Reef Tank Flow Calculator

A reef tank flow calculator determines the ideal water turnover rate (gallons per hour) for your reef tank based on volume and coral type. Enter your tank size, sump, coral mix, and tank length to get a recommended GPH range, powerhead count, and placement guidance.

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Tank Volume

Enter your display and sump capacity

Display Tank (gal)

Sump (gal) - optional

Coral Type / Tank Style

Different corals demand very different flow rates

Tank Length

Used to suggest how many powerheads to run

Tank Length (in)

Recommended Flow

95 gal

360 L

Gross volume

85.5 gal

324 L

Net water (-10%)

20-40x

Mixed

Turnover range

48 in tank

Powerheads

Recommended total flow rate

1,710 - 3,420 GPH

85.5 gal net × 20-40x turnover

Combine return pump + powerheads to hit this range. Most reefers skew toward the high end.

Suggested powerhead sizing

2 powerheads - ~1,283 GPH each

Two smaller powerheads in opposing positions almost always beat a single large one - you get better mixing and fewer dead spots.

Placement tips

Aim for varied flow zones - higher turbulence in the upper third for SPS frags, calmer pockets in the lower third for LPS. Two opposing powerheads at opposite ends of the tank work best.

Flow pattern

Random / gyre - alternating opposing flow keeps detritus suspended.

Reef Tank Flow Tips

✓Random beats steady. Corals evolved in turbulent reef crests where flow direction changes constantly. A wave timer or controllable powerheads on alternating modes outperform a single fixed pump.
✓Don't blast individual corals. Tissue recession and STN often trace back to a powerhead pointed too directly. Aim past corals, not at them.
✓Watch the sand bed. If your sand starts to scour, raise your powerheads or angle them upward. Sandstorms damage filter feeders and can sandblast corals.
✓Mind dead spots behind rock. Detritus settles wherever flow doesn't reach, fueling nuisance algae and dinos. Place a small auxiliary pump if needed.
✓Return pump flow counts. Don't forget to add your return pump's actual head-loss-adjusted GPH when totaling system flow - it can be a third of your turnover on its own.

How It Works

1Sum display + sump volume. The calculator adds display and sump gallons, then subtracts ~10% for rock and sand displacement to get net water volume.
2Apply a turnover multiplier. Soft/LPS targets 10-20x, mixed reef 20-40x, SPS 40-60x, FOWLR 10-15x. Net volume × this range gives total flow GPH.
3Suggest powerhead count. Tank length drives the count - one powerhead per 24-30 inches. The midpoint flow is split evenly across them for per-powerhead GPH.
4Recommend placement and pattern. Each coral type gets specific placement guidance - opposing flow high in the tank for SPS, indirect flow for soft/LPS, a single high-mounted wave-maker for nanos.

Frequently Asked Questions

How much flow does a reef tank need?

Reef tank flow is measured as turnover - total system gallons per hour divided by net water volume. Soft coral and LPS tanks target 10-20x turnover, mixed reefs target 20-40x, and SPS dominant tanks target 40-60x. A 75 gallon mixed reef with a 20 gallon sump (about 85 gallons net) wants roughly 1,700 to 3,400 GPH of total flow from the return pump and powerheads combined.

What is turnover rate in a reef aquarium?

Turnover is how many times per hour the entire water volume circulates through pumps and powerheads. A 100 gallon system with 3,000 GPH of total flow has a 30x turnover. Higher turnover means faster gas exchange, better waste suspension, and more vigorous coral health, but also more risk of sandstorms and tissue damage if flow is aimed directly at sensitive corals.

How many powerheads do I need for a 75 gallon reef tank?

A standard 75 gallon reef tank is 48 inches long, which calls for two powerheads in opposing positions. Aim for a combined powerhead GPH of roughly 1,500 to 2,500 for a mixed reef, split across the two pumps. Two smaller powerheads beat one large one - you get more random mixing, fewer dead zones, and built-in redundancy if one pump fails.

Is too much flow bad for corals?

Yes, excessive direct flow can cause tissue recession, slow tissue necrosis (STN), and sandstorms that damage filter feeders. The fix is rarely to lower total flow - instead, redirect powerheads so they don't blast corals directly. Aim across the tank, not at it, and use random or wave modes so the flow is variable rather than constant on the same spot.

How do I calculate GPH for my reef tank?

Start with total water volume: display gallons plus sump gallons, minus about 10 percent for rock and sand displacement. Multiply that net volume by your target turnover - 10-20x for soft/LPS, 20-40x for mixed reef, 40-60x for SPS. The result is your recommended total flow in gallons per hour. Add the rated GPH of your return pump and all powerheads together, accounting for head loss on the return.

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