Short answer
Watch for tissue recession from the base or edges, pale or bleached coloration (zooxanthellae loss), prolonged closed polyps in normal flow, sloughing tissue or visible skeleton, brown jelly or string-like waste, and lack of polyp extension during normal photoperiod. Catching these early gives you 24-72 hours to act before the colony fails.
In depth
Coral does not die suddenly in most cases. There is a window of warning signs, and reefers who learn to read them save colonies that less experienced keepers lose.
Visual warning signs
- Tissue recession: white skeleton appearing at the base or branch tips. RTN (rapid) progresses in hours; STN (slow) progresses over days.
- Bleaching: coral turns pale or stark white as it expels its zooxanthellae. Often caused by lighting that is too bright, temperature swings, or alkalinity stress.
- Closed polyps for days: a normal coral extends polyps during the photoperiod. Permanent retraction in stable flow is a sign of stress.
- Brown jelly: a stringy brown bacterial slime that spreads from one coral to neighbors. Usually fatal once visible.
- Skeleton showing through tissue: tissue thinning to the point where the underlying skeleton is visible. The coral is starving energetically.
What to do when you see the signs
- Test all parameters immediately: salinity, alkalinity, calcium, magnesium, ammonia, nitrate, phosphate, temperature.
- Check for predators or pests: AEFW on Acropora, red bugs, Aiptasia stinging neighbors, fish nipping the coral.
- Move the coral: lower light or different flow may stop the decline.
- Frag and save: for SPS, cut the healthy portions and try to save fragments before the entire colony fails.
- Pull the ICP: if you cannot find an obvious cause, send water for ICP testing - heavy-metal contamination is the silent killer no hobbyist test detects.
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