What should I do if aquarium coral is bleaching?

Why this happens

Corals can pale or turn white after heat, excessive light, salinity shock, chemical exposure, starvation, disease or severe instability. Bleached tissue may remain alive but has reduced energy supply.

• A heater fault can raise temperature rapidly.
• New lighting can exceed the coral’s acclimated intensity.
• Low or rapidly changing nutrients may contribute stress.
• Tissue recession can expose white skeleton and be mistaken for bleaching.

Look closely for living translucent tissue and polyp response. Exposed clean skeleton has a different prognosis from pale tissue still covering the colony.

What to check first

• Did temperature or lighting change recently?
• Is tissue still present over the skeleton?
• Are multiple coral species affected?
• Are salinity, alkalinity and nutrients stable?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Stop the acute stressorCorrect overheating, dosing faults or excessive light without creating another shock.
2. 2. Support stable conditionsMaintain oxygen, salinity and alkalinity and avoid unnecessary handling.
3. 3. Reduce light carefullyUse shading or intensity reduction rather than sudden prolonged darkness.
4. 4. Monitor tissue and feedingFollow species-appropriate feeding only when the coral can respond.

How to interpret the response

Colour recovery can take weeks or months. Stabilisation of tissue and normal polyp behaviour is an earlier positive sign than rapid return of pigment.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not assume white means dead, apply multiple dips, make large nutrient swings, overfeed a non-responsive coral or expose it to abrupt darkness and full light cycles.

When it is urgent

Rapid tissue loss, widespread bleaching, heater failure, chemical contamination or fish and invertebrate distress is urgent.

How to prevent it next time

Use temperature alarms, light acclimation, calibrated dosing and quarantine while keeping consistent photographs and parameter trends.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.