Aquarium Flow Calculator — Turnover, Return Pump, Sump Drain-Back & Wavemaker Planner

For a community tropical tank, 4–5× the display volume per hour is the standard rule of thumb — a 75-gallon tank wants roughly 300–375 GPH. Planted tanks do well at 4–6×, while cichlids and goldfish with heavy bioloads need 8–10×. Enter your tank size in the Turnover Planner tab above for exact GPH and LPH targets.

It depends what you mean by "turnover". Water flowing through the sump only needs 5–10× per hour. In-tank circulation from wavemakers is much higher: 10–20× for soft corals, 15–30× for LPS, and 30–50×+ for SPS. Mixing the two up is the most common reef flow mistake — the planner and wavemaker tabs keep them separate.

Multiply your display volume by your turnover target, then divide by 0.7 to get the rated size — media, head height and dirty pads typically cost 20–40% of a filter's bench rating. Example: a 75-gallon community tank at 4–5× needs about 430–535 GPH rated. The Filter Sizer tab does this math for you.

Size for 5–10× your display volume per hour through the sump, then correct for head loss: vertical lift plus roughly 0.75 ft per 90° elbow, 1.5 ft per check valve and 0.5 ft per 10 ft of horizontal pipe. A 75-gallon tank at 7× with 4 ft of lift and typical fittings needs about a 750 GPH rated pump — but always confirm against the pump's flow curve at your head height.

Add the vertical lift (sump water surface to display water surface) to fitting losses: ~0.75 ft per 90° elbow, ~0.375 ft per 45° elbow, ~0.5 ft per ball valve, ~1.5 ft per check valve, ~0.25 ft per union, ~0.5 ft per 10 ft of horizontal run. These are rough rules of thumb, not engineering data — the manufacturer's flow curve is the authority. The Return Pump Sizer tab totals them for you.

It depends on whether the drain-back volume exceeds the sump's free space. When the pump stops, water above the overflow weir plus water sitting in the drain plumbing falls into the sump. Use the Power-Outage Drain-Back Checker tab: enter your display and sump dimensions and it computes the margin. Then verify by unplugging the pump — the calculator is an estimate, the unplug test is the truth.

Drain-back = (display length × width × drop from water surface to overflow weir ÷ 231) + (π × (pipe diameter/2)² × pipe length ÷ 231), in US gallons. A 48×18 inch tank dropping 1.5 inches sends back about 5.6 gallons from the display alone. The drain-back tab computes both parts for you.

Four levers: (1) lower the sump's normal operating water level to leave more free space, (2) reduce the display drop by raising the overflow weir, (3) drill a small siphon-break hole in the return line just below the water surface so the return line can't back-siphon, (4) use a bigger sump. Do not rely on a check valve alone — they stick and fail.

A siphon break (anti-siphon) hole is a small hole — typically 1/8 inch — drilled in the return line just below the display tank's normal water surface. When the pump stops, air enters the hole and breaks the siphon, so only a tiny amount of water drains back through the return line. Keep it clear of algae and check it regularly.

Baffles set your sump's operating water level: water flows over the tallest baffle, so the normal level sits just below it. Keep the operating level low enough that the drain-back volume (from the checker tab) fits with at least 20% spare capacity below the rim. A common setup keeps 1–2 inches of freeboard above the operating level as a minimum, plus whatever the drain-back math demands.

As a starting point: two pumps for tanks up to 4 feet, three up to 6 feet, four for longer tanks — opposite placement beats one giant pump for coverage and redundancy. The Wavemaker Placement tab splits your circulation target across the recommended count and shows per-pump GPH.

Aim pumps at each other from opposing corners rather than directly at corals — colliding flows create the turbulent, varied currents corals like. Angle them slightly upward for surface agitation (gas exchange), keep intakes away from anemones and small fish, and run alternating pulse modes if your pumps support it.

Turnover is water passing through filtration (sump or canister) per hour — 4–10× is typical. Circulation is total water movement inside the tank, including wavemakers — 10–50×+ in reefs. A reef can have 40× circulation with only 7× going through the sump; both numbers matter, for different reasons.

Multiply GPH by 3.78541 to get liters per hour. So 500 GPH ≈ 1,893 LPH. Every result on this page shows both units automatically.

Yes, and many reefers do: you get redundancy (one failure doesn't stop all flow) and finer tuning. The trade-off is cost, space, and slightly lower efficiency per watt. Size each pump for roughly half the needed flow at your head height, and plumb them so one can't back-feed the other when off.

Yes. This tool runs 100% in your browser — your tank dimensions, sump measurements and flow targets never leave your device. There are no accounts, no uploads and no external requests.