Coral Bleaching in the Aquarium: Causes and Recovery
Coral bleaching is the loss of the symbiotic algae that feed a coral, leaving it pale or white but still alive. Learn the aquarium triggers — heat, light and parameter swings — how bleaching differs from tissue necrosis, and how corals recover once the stress is removed.
What bleaching is
Corals live in symbiosis with photosynthetic dinoflagellates (zooxanthellae). Under environmental stress this symbiosis breaks down and the coral loses or expels its algae, exposing the white calcium-carbonate skeleton through the now-transparent tissue — and the coral begins to starve. Crucially, in bleaching the tissue is still present; the coral is alive but starving.
Aquarium triggers
Heat stress is the classic trigger: water as little as 1 to 2°C above a coral's normal tolerance can cause bleaching. NOAA quantifies accumulated heat stress as Degree Heating Weeks (DHW): above 4°C-weeks causes significant bleaching, and above 8°C-weeks causes severe bleaching and mortality. In the aquarium, temperature spikes, overly intense or rapidly increased lighting, and swings in alkalinity or salinity are common causes. Under moderate heat stress, corals actively expel photo-damaged symbionts to limit oxidative stress.
Helping corals recover
- Find and remove the stressor — most often, check for a temperature spike or a recent lighting increase.
- Stabilise temperature, alkalinity and salinity, and avoid further swings.
- Reduce light intensity or photoperiod if lighting was the trigger, then increase it slowly.
- Support the starving coral with feeding while symbionts are re-acquired.
- Be patient — recovery depends on relieving stress before the coral starves.
Sources: coralreefwatch.noaa.gov ; pmc.ncbi.nlm.nih.gov