A coral turning white is bleaching: it’s losing the algae living in its tissue, which give it colour and most of its food. In home tanks the usual causes are heat, a sudden increase in light, and changes in salinity or water chemistry. Find and fix the cause quickly; a bleached coral can recover if it keeps its tissue.
Bleached or dead?
A bleached coral is white but still has living tissue over its skeleton: you can see polyps, and the surface looks soft. A dead coral shows bare, often fuzzy skeleton. Tissue peeling away from the base or tips is something else; see RTN and STN.
The usual causes
Seneye lists the causes of bleaching: temperature changes (usually overheating), too much light, changes in salinity, changes in water chemistry, starvation and some others that apply mainly in the wild.
- Heat. A stuck heater or a hot spell. Charterhouse Aquatics says anything above 28 °C needs dealing with straight away. See reef tank too hot.
- More light, suddenly. A new fitting, a higher setting or moving a coral up the rockwork. Corals need to be brought up to stronger light gradually.
- Salinity changes. Often from a failed top-off or topping up with saltwater.
- Very low nutrients. Corals starved of nitrate and phosphate can pale. See zero nitrate and phosphate.
What to do
Find the cause and fix it before anything else.
- Check temperature on a second thermometer, and look at your log for recent spikes
- If you changed the lighting, turn it back down and raise it gradually over a few weeks
- Check salinity with a calibrated refractometer
- Test nitrate and phosphate; ease off nutrient removal if they’re near zero
- Leave the coral where it is unless it’s clearly in too much light
Recovery
Seneye notes that a bleached coral can recover, but it’s a struggle: without its algae, it has to find food another way. Steady conditions give it the best chance. Many keepers also offer coral food while it recovers.
Log your temperature and tests through the recovery. If colour slowly returns over weeks, you’ve found the cause.
Common questions
Can a bleached coral recover?
Yes, if it keeps its living tissue and conditions are fixed quickly.
Why is only one coral bleaching?
It may be in the strongest light, the warmest spot or more sensitive than its neighbours. Check light first.
Is a white coral always dead?
No. White with tissue is bleached; bare skeleton is dead.
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.
- Seneye: what causes coral bleaching?
- Charterhouse Aquatics: heatwave survival guide
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
Keep going
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