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- Corals need moving water to swap oxygen, carbon dioxide, and nutrients through the thin, still layer of water on their surface. In lab studies, more flow meant faster growth and better survival under heat stress.
- A common hobby starting point is total in-tank flow of about 10–20× the display volume per hour for soft corals, 20–30× for a mixed reef, and 30–50× for SPS. That is a rule of thumb, not a research result.
- Varied flow from two or more powerheads, with no dead spots and no nozzle blasting a coral up close, matters more than the exact number.
Why corals need flow
Every coral is wrapped in a thin layer of water that barely moves, called the diffusive boundary layer. Oxygen, carbon dioxide, and nutrients have to cross it by slow diffusion. Moving water over the coral makes that layer thinner, so gases and nutrients move in and out faster.
Shashar and colleagues (1993) measured this directly with tiny oxygen probes. Inside the boundary layer of the massive coral Favia favus, oxygen swung from 373% of air saturation in the light to zero at night, even though the surrounding water stayed constant. A 5 cm/s current (about 2 inches per second) thinned the layer from 2.44 mm to 1.90 mm, letting oxygen escape and enter faster.
Patterson, Sebens, and Olson (1991) measured the Caribbean reef-builder Montastraea annularis in situ, from an underwater lab, and found that both photosynthesis and dark respiration rose as water motion increased. In other words, a coral in still water is partly starved of the exchange it needs, day and night.
What the studies found
| Study | Coral | Flow tested | Result |
|---|---|---|---|
| Schutter et al. 2010 | Galaxea fascicularis | 0, 10, 20, 25 cm/s for 42 weeks | No flow gave significantly lower growth; 25 cm/s gave the highest growth |
| Nakamura & van Woesik 2001 | Acropora digitifera | 2–3 vs 50–70 cm/s, at high temperature and light | High survival in high flow, low survival in low flow; no deaths below 30 °C in either |
| Nakamura, Yamasaki & van Woesik 2003 | Stylophora pistillata (partly bleached) | ≤3 vs 20 cm/s for 7 weeks | Colonies recovered algae and color at 20 cm/s; low-flow colonies stayed pale |
| Shashar et al. 1993 | Favia favus, Stylophora pistillata | Still vs 5 cm/s | Boundary layer 2.44 mm thinned to 1.90 mm; faster oxygen exchange |
The bleaching results matter for home tanks. Nakamura and van Woesik suggest that part of the patchy coral death in the 1998 bleaching event came from differences in local water flow, because flow speeds up the removal of harmful by-products that build up in heat-stressed tissue. So flow is not a cure for overheating, but it is one more reason to keep pumps running and to fix a stuck heater fast (see our reef tank temperature guide and coral bleaching science).
One caution: these studies used steady flow speeds measured at the coral, in centimeters per second. Hobby turnover numbers are pump ratings in gallons per hour. They are related but not the same, and no study has turned "×30 turnover" into a coral-health result.
How much turnover to aim for
Turnover is how many times per hour your pumps could move the display's volume. Reefers usually count the pumps' rated flow and add the powerheads together. These are common hobby starting points, not research numbers:
- Soft corals and many LPS: about 10–20× per hour, gentler and more diffuse.
- Mixed reef: about 20–30× per hour.
- SPS-dominated: about 30–50× per hour or more, strong and varied.
| Display volume | 10× | 20× | 30× | 50× |
|---|---|---|---|---|
| 20 US gal (76 L) | 200 GPH | 400 GPH | 600 GPH | 1,000 GPH |
| 40 US gal (151 L) | 400 GPH | 800 GPH | 1,200 GPH | 2,000 GPH |
| 75 US gal (284 L) | 750 GPH | 1,500 GPH | 2,250 GPH | 3,750 GPH |
| 120 US gal (454 L) | 1,200 GPH | 2,400 GPH | 3,600 GPH | 6,000 GPH |
Turnover (GPH) = display volume (US gal) × turnover multiple. A 40-gallon mixed reef at 25× needs about 40 × 25 = 1,000 GPH of rated pump flow, for example two powerheads of about 500 GPH each. Not sure of your volume? Use the reef tank volume calculator.
The return pump
If you have a sump, the return pump adds some flow, but most reefers keep it modest (often quoted as about 3–10× the display volume per hour) so the skimmer and filter socks have time to work. Pumps are rated at zero head, and real flow drops as you pump higher, so read the manufacturer's flow curve at your plumbing height rather than the box number.
Where to put powerheads
- Use two or more. Two smaller pumps from different directions reach more surfaces than one big pump and leave you some flow if one fails.
- Aim across the rockwork, not at corals. Point the flow along the glass or across the top of the rock so it spreads out before it reaches the corals.
- Keep the surface moving. Rippling the surface helps oxygen in and carbon dioxide out, which also helps reef tank pH.
- Reach behind and under the rock. Dead spots behind rockwork collect detritus that feeds algae and cyanobacteria.
- Vary it. Alternating or random modes on a controller, or pumps facing each other, keep the flow from always hitting the same side of a coral.
- Guard the intakes. Cover intakes with foam or a guard so snails, fish, and loose frags are not pulled in.
How to check flow at a coral
You can estimate flow speed without a meter. Drop a small sinking pellet or a pinch of frozen food upstream of a coral and time how far it travels in one second. Ten centimeters (4 inches) in a second is 10 cm/s, the lowest steady speed that still helped growth in the Galaxea study. A nearly still particle (under about 3 cm/s, or just over an inch per second) is the low-flow range where bleached corals stayed pale.
After feeding, watch where food and detritus settle. Spots that collect debris every time are your dead spots.
Signs of too little or too much flow
| Too little flow | Too much flow |
|---|---|
| Detritus piles in the same spots | Polyps stay retracted on the side facing the pump |
| Cyano or film algae in still corners | Fleshy LPS tissue torn or pulled back from the skeleton |
| Oily film on the water surface | Sand blowing and piling up, exposing the bottom |
| Corals healthy on one side, bare on the sheltered side | Soft corals and anemones constantly bent flat or moving to escape |
These signs have other causes too. Detritus and cyano also point to nutrients (see algae outbreaks and water testing), and receding tissue can be STN or RTN. Change one thing at a time and watch for a week.
Flow gear we would start with
We list these by name only for now. See our wavemaker and powerhead guide for the trade-offs.
- Controllable propeller powerheads: EcoTech Marine VorTech or AquaIllumination Nero, for wide, turbulent flow with random and pulse modes.
- Gyre-style pumps: Maxspect Gyre, for a broad sheet of flow that suits many LPS tanks.
- Nano tanks: Tunze Turbelle Nanostream, a compact, long-running option.
Flow during a power outage
When the power goes out, corals and fish lose flow and gas exchange together. A battery air pump or a battery-backed powerhead keeps water moving and the surface broken. Our temperature guide covers outages in more detail.
Common flow mistakes
- One big pump aimed straight at the rockwork.
- Trusting the box rating of a return pump without checking head height.
- Placing a new coral right in front of a nozzle.
- Turning flow way up overnight on an established tank instead of in steps.
- Letting powerheads clog with coralline algae until flow quietly drops. Clean them every few months.
FAQ
How much flow does a reef tank need? There is no research-based turnover number. A common hobby rule of thumb is total in-tank flow of about 10 to 20 times the display volume per hour for soft corals, about 20 to 30 times for a mixed reef, and about 30 to 50 times for SPS-dominated tanks, counted from the pumps' rated flow. Use it as a starting point, then adjust by watching your corals and where detritus settles.
Does the return pump count toward reef tank flow? Yes, but usually only a little. Most reefers run the sump return at a modest rate (often cited as about 3 to 10 times the display volume per hour) so the skimmer and filter socks can keep up, and get most in-tank flow from powerheads. Return pumps are rated at zero head, so the real flow at your plumbing height is lower; check the pump's flow curve.
Can corals get too much flow? Yes. Flow aimed straight at a coral at close range can keep polyps retracted and, in fleshy LPS, tear or strip tissue. Lab studies found faster flow generally helped growth and heat tolerance, but those were steady speeds of roughly 10 to 70 cm/s, not a pump nozzle a few inches away. Aim for flow that moves the coral gently rather than blasting it.
Is random or alternating flow better than steady flow? Most reefers prefer varied flow because it reaches more surfaces and leaves fewer dead spots, and wild reefs have wave-driven, oscillating flow. Controlled studies comparing pump modes in home-size tanks are scarce, so treat this as good practice rather than proven. The goal is that every part of the rockwork sees some movement.
What are signs of low flow in a reef tank? Detritus piles that come back in the same spots, cyanobacteria mats in still corners, a film on the water surface, and corals that are healthy on the flow side but bare or algae-covered on the sheltered side. These signs also have other causes, so check nutrients and lighting too.
References
- Shashar, N., Cohen, Y. & Loya, Y. (1993). Extreme diel fluctuations of oxygen in diffusive boundary layers surrounding stony corals. Biological Bulletin 185: 455–461. doi:10.2307/1542485
- Patterson, M.R., Sebens, K.P. & Olson, R.R. (1991). In situ measurements of flow effects on primary production and dark respiration in reef corals. Limnology and Oceanography 36(5): 936–948. doi:10.4319/lo.1991.36.5.0936
- Nakamura, T. & van Woesik, R. (2001). Water-flow rates and passive diffusion partially explain differential survival of corals during the 1998 bleaching event. Marine Ecology Progress Series 212: 301–304. doi:10.3354/meps212301
- Nakamura, T., Yamasaki, H. & van Woesik, R. (2003). Water flow facilitates recovery from bleaching in the coral Stylophora pistillata. Marine Ecology Progress Series 256: 287–291. doi:10.3354/meps256287
- Schutter, M., Crocker, J., Paijmans, A., Janse, M., Osinga, R., Verreth, A.J. & Wijffels, R.H. (2010). The effect of different flow regimes on the growth and metabolic rates of the scleractinian coral Galaxea fascicularis. Coral Reefs 29: 737–748. doi:10.1007/s00338-010-0617-2