What bleaching actually is
Corals get their color and much of their food from zooxanthellae — symbiotic algae living in their tissue. Under stress, the coral expels that algae. Without it, the clear tissue shows the white skeleton underneath. That's bleaching. The coral isn't dead yet: if you fix the cause quickly, it can recolor and recover. Left alone, a bleached coral starves.
The most common causes in a home reef
- Temperature swings. Too hot (a summer heat wave, a stuck heater) is the classic trigger. Rapid changes stress corals as much as the wrong absolute temperature.
- Light shock. A new light, a jumped intensity setting, or a coral moved higher in the tank can photo-bleach it in days. New corals need to be acclimated to light gradually.
- Parameter swings. A fast shift in alkalinity is a frequent culprit — an "alk burn" bleaches tips first. Salinity and pH swings do it too.
- Nutrients too low. Nitrate and phosphate bottomed out to zero starves the zooxanthellae and pales corals from the base up.
- Flow and pests. Too little flow, or pests and disease irritating the tissue, can cause localized bleaching or recession.
How to diagnose it
Work from most-likely to least. Check your thermometer and heater first, then review your recent parameter logs for any sudden change — especially alkalinity. Did you add a light, move the coral, or change a dosing amount in the last week? Bleaching almost always traces back to something that shifted recently, which is why a dated log is so valuable.
How to fix it
- Stabilize temperature first — get it back into the 76–78 °F band and hold it steady.
- Stop the swing. If alkalinity or another parameter moved fast, ease it back slowly toward the normal range — don't overcorrect.
- Reduce light if it's likely light shock; lower intensity or move the coral down, then raise it back gradually.
- Feed the tank if nutrients are at zero — let nitrate and phosphate come up into range.
- Be patient. Keep everything stable and give it weeks. Recovering corals recolor slowly from the edges in.
Catch it earlier next time
Most bleaching is preventable — it's a swing you didn't notice in time. With TankMate you can snap a photo of the coral for an AI health read on whether it's bleaching, recession, or something else, and it factors in your logged water values for a sharper answer. Log your parameters and the app flags the drift before your corals do. It's free on Google Play.