Troubleshooting: Corals
Why are my corals not opening?
Polyps stay retracted for a day or more, a soft coral shrinks, or corals pale or brown over a couple of weeks.
Last checked . How we check
What should I check first?
- Salinity, with a refractometer calibrated in 35 ppt fluid, not RO water.
- Temperature, on a second thermometer.
- Look closely at night with a torch for pests, and watch for fish or crabs picking at it.
- Anything changed in the last two weeks: new light settings, new coral, moved rock, new carbon, a big water change, new salt.
- Alkalinity, compared with your last few readings.
What is most likely causing it?
Most likely first. The order is our reading of how often the sources link each cause to this symptom.
- Salinity off, often from a top-off failure or a refractometer out of calibration. Every coral is affected at once. See ATO failure.
- Temperature off. All corals react together; check the heater and room temperature.
- Pests or nipping fish. One coral or one type only; bite marks, small flatworms or nudibranchs at night. See pests and hitchhikers.
- Light changed too fast. Paling or whitening, polyps not extending or edges curling after a new light or new settings (LiveAquaria).
- An alkalinity swing. Recent readings moved by more than about 1 dKH in a day. See alkalinity swings.
- Nutrients too low. Pale corals, SPS polyps retracted and no growth with nitrate and phosphate at zero (Reef Builders, Fauna Marin, BRS).
- A neighbouring coral or a new soft coral releasing chemicals. Started after adding a leather or a coral placed close by.
What should I do?
- Correct salinity or temperature first if either is off, slowly.
- If you changed the lights, go back to the old settings, then raise them over three to four weeks (LiveAquaria).
- Run fresh activated carbon and do a normal water change if a new coral or a chemical event is suspected.
- Remove pests by hand and dip the coral outside the tank. See pests and hitchhikers.
- If nutrients are at zero, let them rise to measurable levels.
- Give it a few days: many corals close for a day or two after a change and then recover.
What not to do
- Changing several things at once: you will not know what helped.
- Moving the coral every day.
When should I get help?
Something dying right now? Go to the emergency playbooks.
Where do sources disagree?
| Topic | What sources say | Our reading |
|---|---|---|
| What to check first |
| Salinity and temperature are quick, cheap and affect everything, so we start there. |