Corals usually stay closed because something changed: salinity, temperature, alkalinity, light, flow, a new tankmate or a neighbour stinging them. Check salinity and temperature first, then alkalinity, then look closely for pests and nipping fish. A newly added coral may simply need a few days to settle.
Check these first
Frag Box Corals, a coral shop, says incorrect salinity is the most common mistake it sees. Work through the quick checks before anything else.
- Salinity. With a calibrated refractometer. See reef salinity and how to calibrate a refractometer.
- Temperature. On a second thermometer. Frag Box suggests 76–78 °F (24.4–25.6 °C).
- Alkalinity. Compare with your last few readings. A sudden move matters more than the number.
- Equipment. Is the return pump running? Has a wavemaker stopped or been moved?
Then look closely at the coral
Use a torch after lights-out, when many pests are active.
- Nibbling. Frag Box singles out angelfish, but other fish, crabs and shrimp can nip too.
- Pests. Look for flatworms, nudibranchs or spiders on the coral and its base.
- Neighbours. A coral within reach of another’s sweeper tentacles may stay shut.
- Light. A new light, a moved coral or a changed schedule can make corals close for days.
- Flow. Too much can keep soft corals shut; too little can leave them covered in detritus.
New corals need time
A newly added coral often stays closed for a few days while it adjusts to new light, flow and water. Avoid moving it repeatedly in that time. If it stays closed for more than a week, run through the checks above.
Some corals close for a day or two to shed a waxy layer. If it peels off and the coral opens again, that’s normal.
Change one thing at a time
Once you have a likely cause, change one thing and give it a few days. Log your tests and note what you changed. If several corals close at once, treat it as a water problem first; see reef tank problems.
Common questions
How long can a coral stay closed?
A few days is common after a move or a change. If it stays closed beyond a week, or starts losing tissue, look harder.
Why are all my corals closed?
Several at once points to the water: salinity, temperature, alkalinity, or something added to the tank.
Should I move a closed coral?
Not straight away. Check the water and pests first; moving it adds another change.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. It can’t see your tank: if livestock is in distress, act first, then ask an experienced local shop or club, ideally with your test log to hand.
- Frag Box Corals: corals not opening
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
- Bulk Reef Supply: how to use a refractometer
Keep going
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