Coral bleaching — causes and recovery in a home reef

DiagnosticRecent heater failure, AC failure, or summer heat spike. Bleaching on top surfaces of SPS first, then LPS. Coincides with stressed fish.

FixStabilize temperature at 77–78 °F (25 °C). Use redundant heaters (two at half wattage each — see /calculators/heater-wattage). Add a fan + chiller for summer if ambient room temp exceeds 80 °F.

DiagnosticBleaching top-down on SPS within 2 weeks of a new light, raised intensity, or moved coral up in the tank.

FixReduce intensity by 30% and ramp back up over 4 weeks. Acclimate coral by placing it on the substrate first and moving up 3–4 inches per week.

DiagnosticRecent alkalinity test shows >1.4 dKH change. Often after doser miscalibration or a water change with high-alk salt mix into a low-alk tank.

FixTest daily, stabilize at your previous baseline (not at 8 dKH if you'd been running 11), and don't change alk by more than 0.5 dKH per day. Coral mucus production is the most alk-shock-sensitive process.

DiagnosticRecent large water change (>30%), or ATO failure dropping/raising salinity. Refractometer reads outside 1.024–1.026 SG.

FixSlowly correct salinity at ≤0.001 SG per day. ATO failsafe sensors prevent the recurrence — install one.

DiagnosticAggressive carbon dosing, sudden phosphate-remover overdose (GFO), or starting GFO and a scrubber simultaneously. Bleaching diffuse across the tank.

FixBack off carbon dosing or GFO. Target NO3 5–10 ppm and PO4 0.03–0.08 ppm. Many SPS bleach below NO3 1 ppm or PO4 0.02.

DiagnosticBleaching limited to one species. Recently added stock from a new source. Acropora-Eating Flatworms (AEFW) visible under dip, or red-band/brown-jelly disease.

FixDip new SPS (CoralRX, Bayer) before adding. For active infections: frag healthy tips, dispose of infected colony.