Flow: The Parameter Nobody Tests
You can test alkalinity to two decimals and still lose corals to a dead spot you never noticed. Flow moves food in, moves waste out, and stops detritus from settling on tissue. There is no test kit for it, so you read it with your eyes.
What good flow looks like
- Fleshy LPS sway gently. A torch whipping back and forth is receiving too much.
- SPS branches have no film or fuzz on them, and the polyps are extended during the day.
- Nothing settles anywhere. If you can see a line of detritus behind a rock, that is a dead spot and it will become a cyano patch.
- The surface has visible ripple, because gas exchange happens there.
Turnover and why the number is a lie
The old rule of twenty to forty times tank volume per hour is a rough starting point and nothing more. Two pumps aimed at each other in a 40 gallon tank can produce a beautiful random field, while a single powerful pump can produce a jet and a dead corner in the same volume. What you want is random, turbulent, changing flow, not a fire hose.
Placing pumps
- Aim pumps at each other or at the front glass, never directly at a coral.
- Use a wave or pulse mode if your pump has one. A changing pattern reaches places a fixed jet never will.
- Keep a pump off the sand bed unless you enjoy a sandstorm.
- Cover every intake with a guard before an anemone goes in the tank. That single omission is the most common way to lose a bubble tip.
Reading the tank
Turn the return off and squirt a small cloud of food or fine sand near a rock, then turn the pumps back on and watch where it goes. Anywhere the cloud lingers is a spot where waste will sit. Do this once a season and after every rescape.
By coral group
| Group | Flow | Sign it is wrong |
|---|---|---|
| Acropora, Seriatopora | High, turbulent | Dying from the base outward, fuzz on branches |
| Montipora | Moderate to high | Detritus collecting in a plate cup |
| Euphyllia, acans, scolymia, chalices | Low | Tentacles retracted all day, torn flesh |
| Zoas and palys | Moderate | Closed polyps with grit between them |
| Leathers, xenia, GSP | Moderate | Limp, no pulse, no polyp extension |