Troubleshooting

Reef tank troubleshooting

Diagnose by symptom. Each page lists causes in order of how often they actually happen, with a quick diagnostic and a real fix for each.

Cloudy aquarium water — causes and fixes
Symptom: The water is cloudy / hazy / milky / green / discolored.
High nitrate (NO₃⁻) in a reef tank — diagnosis and fixes
Symptom: Nitrate reading >20 ppm in a reef tank. Brown algae, hair algae, or dinoflagellate blooms. Cyano spots.
Low alkalinity in a reef tank — diagnosis and fixes
Symptom: Alkalinity below 7 dKH. SPS tips losing color (often the first sign). pH unstable. Calcium also dropping.
Reef tank algae identification — what is this and how do I fix it?
Symptom: Something green/brown/red is growing in your tank that you don't want.
Coral bleaching — causes and recovery in a home reef
Symptom: Coral has lost color — paled, gone white, or its tissue has receded.
Reef tank pH swings — diagnosis and fixes
Symptom: pH below 7.9, above 8.4, or swings more than 0.3 units day to night.

Last reviewed 2026-05-25