How much flow does my reef tank need?
By what you keep: 20–30 tank volumes an hour for soft corals and LPS, 30–40 for a mixed reef, 40–60 for SPS, 10–20 for a fish-only tank with rock. Those are the middle of the ranges the retailers publish — Bulk Reef Supply's 20–40, Swell's 20 and 50, Reef Tank Resource's 20–40 and 40–100 — and they are wide on purpose, because what a coral wants is a speed at the coral, not a volume in the tank, which is the second half of this page. A standard 120 gallon mixed reef (434 L as kept) wants 3,450–4,600 GPH of total circulation — call it 4,000 GPH. The chart runs every standard size three ways.
| Tank | Softies & LPS (20–30×) | Mixed (30–40×) | SPS (40–60×) | Pumps |
|---|---|---|---|---|
| 5.5 gallon | 100–150 GPH | 150–200 GPH | 200–300 GPH | 1 |
| 10 gallon | 190–290 GPH | 290–380 GPH | 380–570 GPH | 1 |
| 15 gallon | 270–410 GPH | 410–550 GPH | 550–820 GPH | 1 |
| 20 gallon long | 340–510 GPH | 510–690 GPH | 690–1,050 GPH | 1 |
| 20 gallon high | 370–560 GPH | 560–750 GPH | 750–1,100 GPH | 1 |
| 29 gallon | 530–790 GPH | 790–1,050 GPH | 1,050–1,600 GPH | 2 |
| 37 gallon | 650–980 GPH | 980–1,300 GPH | 1,300–1,950 GPH | 2 |
| 40 gallon breeder | 840–1,250 GPH | 1,250–1,700 GPH | 1,700–2,500 GPH | 2 |
| 55 gallon | 1,100–1,600 GPH | 1,600–2,150 GPH | 2,150–3,250 GPH | 2 |
| 65 gallon | 1,300–1,950 GPH | 1,950–2,600 GPH | 2,600–3,850 GPH | 2 |
| 75 gallon | 1,500–2,250 GPH | 2,250–3,000 GPH | 3,000–4,500 GPH | 2 |
| 90 gallon | 1,700–2,600 GPH | 2,600–3,450 GPH | 3,450–5,150 GPH | 2 |
| 120 gallon | 2,300–3,450 GPH | 3,450–4,600 GPH | 4,600–6,900 GPH | 2 |
| 125 gallon | 2,250–3,350 GPH | 3,350–4,500 GPH | 4,500–6,750 GPH | 3 |
| 150 gallon | 3,050–4,550 GPH | 4,550–6,050 GPH | 6,050–9,100 GPH | 3 |
| 180 gallon | 3,600–5,400 GPH | 5,400–7,200 GPH | 7,200–10,750 GPH | 3 |
| 210 gallon | 4,200–6,300 GPH | 6,300–8,400 GPH | 8,400–12,550 GPH | 3 |
Total flow entering and moving the display — subtract what your return pump delivers before sizing the powerheads. Water as kept (an inch below the rim over sand). Pumps: one under 21 gal, two above, three from 71 in of length.
The turnover target is water moving through the display, and the return pump's flow enters the display. On the 120 above, a return delivering 1,150 GPH at the outlet (the return pump page's worked example) is already 29 % of the 4,000 GPH target. Size the powerheads to the whole figure and you buy 2,000 GPH apiece; size them to the remainder and it is 1,450 GPH — one model smaller, or the same model run slower. None of the four ranking pages mentions the return at all; the tool subtracts it. (It does not count the sump's own flow, which never reaches a coral.)
Turnover is a volume; what corals feel is a speed
"30 times an hour" is the total flow divided by the tank's volume, which makes it a proxy for the thing that matters — how fast water moves past the coral — and a rough one. Run the same 30× through the tank's cross-section and it is 1 cm/s across a 120 gallon (24×24 in end) and 1.5 cm/s across a six-foot 125 (18×21) — the long shallow tank gets 1.5× the water speed from the same turnover, because turnover scales with volume and speed with area, the same mismatch that makes "watts per gallon" wrong for heaters. And both are an order of magnitude under the 10–50 cm/s measured over natural reefs. The mean is not what a coral sees; the jet from the pump is, ten to fifty times faster than the mean and dying away with distance. That is the whole argument for two pumps at two ends and a random pattern: not more total flow, but flow that reaches everything at speed some of the time.
| Tank | 30× is | Mean speed across the end |
|---|---|---|
| 20 gallon high (12×16 in end) | 560 GPH | 0.5 cm/s |
| 40 gallon breeder (18×16 in end) | 1,250 GPH | 0.7 cm/s |
| 75 gallon (18×21 in end) | 2,250 GPH | 1 cm/s |
| 120 gallon (24×24 in end) | 3,450 GPH | 1 cm/s |
| 125 gallon (18×21 in end) | 3,350 GPH | 1.5 cm/s |
| 180 gallon (24×25 in end) | 5,400 GPH | 1.5 cm/s |
Total flow ÷ (width × height), as if every liter crossed the tank once — an order-of-magnitude figure. Natural reef flats and crests: 10–50 cm/s.
How many powerheads, and where?
One in anything under 21 gallons, two from there up, three along a six-footer. Two is the number that matters: a single pump makes one jet and one dead corner behind it; two, at opposite ends, angled across each other, make the whole tank move and let a controller alternate them. Mount them high on the back or end glass — one aimed along the surface for gas exchange, the other angled down across the rock — and never straight at a coral or the sand. Propeller pumps (the MP-, Nero-, Tunze-style) throw a wide, turbulent cone and are what the turnover numbers assume; gyres push a broad sheet along the tank that reverses well and suits long, shallow tanks; the old impeller powerheads make a narrow laminar jet and the least flow per watt. The pattern matters more than the model: random or alternating, never a steady stream on one spot.
Pumps obey the affinity laws — flow with speed, power with speed cubed — so a pump twice the size you need, run at half speed, moves the same water for an eighth of the electricity, quietly, with headroom for the day the corals have grown into the rock. The 120's 2,850 GPH of powerhead flow is about 27 W from modern propeller pumps at full speed; the same flow from two pumps twice the size at half speed is about 7 W. Controllable DC pumps cost more than fixed ones once; a fixed pump that is too big costs more every month.
What does the flow cost to run?
Less than the lights, usually. Modern propeller pumps move about 350–450 L/h per watt at full speed — a Maxspect Gyre XF350 does 5,283 GPH on 52 W, a Tunze 6095 2,510 GPH on 21 W — so the tool's default of 400 L/h per watt puts the 120's powerheads at 27 W, 0.65 kWh a day, all of it ending up as heat in the water. The electricity cost calculator takes those watts and credits them against the heater. Older AC-motor powerheads and any pump throttled by a valve do much worse per liter; if the wattage on the box is the same as a modern DC pump and the flow is half, that is why.
Planted and freshwater tanks: flow is distribution, not turnover
The 4–6× and 8–10× figures for fresh water are the filter's flow — sized on the filter page — and a powerhead is rarely the answer. In a CO₂-injected tank the job is to carry the gas from the diffuser to every leaf and keep the surface gently moving, not churning: a lily pipe or spray bar from the canister does it, and a wavemaker that ripples the surface outgasses the CO₂ you are paying for — the CO₂ calculator shows what a pH-drop lost to surface agitation costs you in ppm. The cases for a freshwater powerhead are specific: river fish (loaches, hillstream species, rainbowfish) that want a current, big cichlids, and a dead corner behind the rock that grows cyanobacteria — the algae ID guide's "low flow, almost every time". Set the tool to planted or freshwater and it says as much instead of printing a powerhead count.
Too much flow, or too little? The signs
Too much looks like corals staying closed or stretched flat, LPS tissue peeled back from the skeleton on the upstream side, a sand storm, fish holding station behind rock, and — in a planted tank — a CO₂ level that will not come up. Too little looks like detritus settling in the same places every week, cyanobacteria in the corners, polyps that never sway, and a film on the surface. Neither is fixed by the total; both are fixed by where the pumps point and whether they alternate. Start at the low end of the band for what you keep, with the pumps on a random mode, and turn them up over weeks while the corals tell you.