Most beginner reef tanks need 15–25× total tank volume per hour in combined flow from the return pump and wavemaker. A 25-gallon tank needs 375–625 GPH of total flow. The exact target depends on what you’re keeping, soft corals and LPS need less than SPS corals, which need significantly more.
Why Flow Is a Filtration Decision, Not Just an Aesthetic One
Water movement determines whether the filtration system works or fails. Biological filtration requires oxygenated water moving continuously through rock surfaces, bacteria in a dead spot with no flow are oxygen-starved and process nothing. Mechanical filtration only captures particles that actually reach the filter media, particles settling in low-flow areas never get there. Protein skimmers require surface agitation to function. Chemical media require water passing through them at an adequate rate.
Flow is also the primary defense against cyano (cyanobacteria) and detritus accumulation. Cyano establishes almost exclusively in dead spots, areas of the sandbed and rockwork where water movement is insufficient to keep the surface oxygenated. A tank with a persistent cyano patch has a flow problem at that specific location, not a chemical problem. More flow to that spot resolves it; chemical treatment without improving flow produces temporary improvement followed by recurrence.
See: Water Flow in Reef Aquariums for the complete flow guide.
Flow Targets by Coral Type
| Coral Type | Flow Target | Flow Character | Examples |
|---|---|---|---|
| Soft corals (mushrooms, leathers) | 10–20× tank volume/hr | Gentle, indirect, randomized, mushrooms close in direct strong flow | Mushroom corals, Kenya tree, toadstool leather, Xenia |
| Soft corals (zoanthids, GSP) | 15–25× tank volume/hr | Moderate, indirect, polyps should wave gently, not fold flat | Zoanthids, green star polyps, button polyps |
| LPS corals | 15–25× tank volume/hr | Moderate, indirect, randomized, sweeper tentacles should wave, not be pinned against the skeleton | Hammer, torch, frogspawn, duncan, brain corals |
| Mixed reef (LPS + easier SPS) | 20–30× tank volume/hr | Moderate to high, randomized, alternating direction prevents tissue damage | Montipora, Pocillopora, easier Acropora + LPS mix |
| SPS dominant | 30–50× tank volume/hr | High, turbulent, randomized, SPS use high flow for nutrient delivery and waste removal from small polyps | Staghorn Acropora, Millepora, Seriatopora |
Flow Targets by Tank Size (Beginner Soft Coral and LPS)
| Tank Size | Target Flow (20×) | Target Flow (25×) | Typical Setup |
|---|---|---|---|
| 10 gallons | 200 GPH | 250 GPH | Return pump (included in AIO) + 1 small wavemaker (Hydor Nano 240) |
| 20 gallons | 400 GPH | 500 GPH | AIO return pump (150–200 GPH) + 1 wavemaker (Jebao SLW-5 or Hydor Koralia 425) |
| 30 gallons | 600 GPH | 750 GPH | AIO return pump (200–300 GPH) + 1–2 wavemakers (Jebao SLW-10) |
| 40 gallons | 800 GPH | 1,000 GPH | Return pump (300 GPH) + 2 wavemakers (Jebao SLW-10 or similar) |
| 75 gallons | 1,500 GPH | 1,875 GPH | Return pump (600–800 GPH) + 2 wavemakers (Jebao SLW-20 or Maxspect Gyre) |
Where to Position the Wavemaker
Flow direction matters as much as flow volume. A 1,000 GPH wavemaker aimed directly at a single corner creates one high-flow zone and leaves the rest of the tank in a dead spot. The same pump positioned correctly creates circulation throughout the tank.
- Mount the wavemaker on the side glass, mid to upper tank, aimed across the tank horizontally, not down at the sandbed or directly at corals
- Angle it slightly downward and toward the front glass, this creates a circular pattern: across the surface, down the front glass, across the bottom, and back
- Position the return nozzle to push flow through the rockwork, not over the top of it, water passing through the rock structure delivers oxygen to biological filtration surfaces
- For tanks over 30 inches long, two wavemakers on opposite sides walls running alternately is more effective than one large unit, alternating flow creates turbulence that reaches more surfaces
The Food Test, How to Verify Flow Coverage
Drop a small pinch of dry food into the tank with all equipment running. Watch where it goes. Food that drifts freely throughout the tank and eventually reaches the filtration intake confirms adequate coverage. Food that immediately sinks and sits in a specific area identifies a dead spot. Adjust the wavemaker angle or add a secondary pump aimed at persistent low-flow areas.
Signs of Too Little Flow vs. Too Much Flow
| Symptom | Too Little Flow | Too Much Flow |
|---|---|---|
| Sand behavior | Detritus settles visibly on sandbed; dark patches in corners | Continuous sandstorm; sand piled against one side of the tank |
| Coral behavior | Corals with tissue recession in areas touching the rockwork; polyps not fully extending | Corals folded flat, retracted, or showing stress during flow peaks; LPS corals deflated |
| Algae pattern | Cyano (red slime) in specific dead spots; detritus accumulation fueling localized algae | Rare, high flow generally suppresses algae growth |
| Skimmer behavior | Skimmer in low-flow sump section producing inconsistent output | Skimmer over-skimming from excessive surface agitation feeding air into pump |
| Fish behavior | Fish avoiding low-flow zones; gathering near powerheads for oxygen | Fish struggling to swim against direct flow; small fish pinned against glass |
Dial In the Flow. Then Verify It.
Set flow to target GPH for your tank size and coral type, position the wavemaker correctly, then run the food test to confirm coverage. Adjust for dead spots. Most flow problems are positioning problems, not volume problems.