How do I know if my coral is dying?

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.

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.