Reef tanks look stable until a problem hits fast. Most issues trace back to water chemistry, flow, or new additions. This guide covers the most common reef tank problems and how to fix them.
Water chemistry swings and testing mistakes
Unstable parameters cause most reef tank trouble. Corals react first. You may see closed polyps or faded color. Fish may breathe faster or hide.
Start with the big four. Keep salinity at 1.025–1.026 specific gravity. Hold temperature at 77–79°F with less than 1°F swing. Maintain alkalinity at 8–9 dKH for mixed reefs. Keep calcium at 400–450 ppm and magnesium at 1250–1400 ppm.
Nutrients need balance, not zero. Aim for nitrate at 2–15 ppm. Keep phosphate at 0.03–0.10 ppm. If both hit zero, corals can pale. If both climb, algae often follows.
Testing errors are common. Old reagents drift over time. Dirty vials skew readings. Always test at the same time of day. Log results for trends. If a number looks wild, retest before reacting.
- Calibrate refractometers with 35 ppt solution, not RO water.
- Mix new saltwater for 12–24 hours with heat and flow.
- Limit alkalinity changes to 0.5 dKH per day.
- Do a 10–15% water change when multiple values drift.
For a stable routine, review our reef tank maintenance schedule. It helps prevent sudden swings. Consistency beats perfect numbers.
Algae blooms, cloudy water, and ugly phases
Algae blooms often start with excess light and nutrients. New tanks also go through ugly phases. Diatoms show as brown dust in weeks 2–6. Green hair algae may follow if nutrients stay high.
First, confirm the source. Test nitrate and phosphate. Check your feeding. One cube of frozen food can push nutrients in small tanks. Rinse frozen foods in a fine net. Reduce photoperiod to 8 hours for whites and 10 hours for blues.
Cloudy water has a few causes. A bacterial bloom looks milky and often follows overfeeding. It can drop oxygen at night. Run the skimmer wet and add surface agitation. A sand storm looks like grit and comes from too much flow on the bed.
Use export tools with restraint. Change filter socks every 2–3 days. Replace carbon monthly. Run GFO only if phosphate stays above 0.15 ppm. Start with half dose to avoid coral shock.
- Target 20–30x total flow for mixed reefs.
- Clean the skimmer neck weekly for steady performance.
- Manually remove algae before it releases spores.
- Add a cleanup crew slowly, based on tank size.
If algae persists, check your source water. Use 0 TDS RO/DI for top-off and mixing. See our RO/DI water guide for a quick checklist.
Fish disease, coral pests, and quarantine gaps
Many reef tank crashes start with a new fish or coral. Ich shows as white spots and flashing. Velvet can look like dust and kills fast. Bacterial infections often show frayed fins and red sores.
Quarantine breaks the cycle. Use a 10–20 gallon bare tank with a sponge filter. Match salinity and temperature to the display. Observe new fish for 2–4 weeks. Keep ammonia at 0 ppm with seeded media and water changes.
Coral pests are also common. Flatworms can irritate LPS and zoas. Aiptasia spreads fast from one hitchhiker. Red bugs target acropora polyps. Dip new corals and inspect plugs under white light.
When something looks wrong, slow down. Confirm symptoms before dosing. Many “reef safe” cures do little. Increase oxygen first during emergencies. Then isolate the affected animal if possible.
- Dip corals for 5–10 minutes, then rinse in clean saltwater.
- Keep a spare heater and air pump for quarantine.
- Do not mix medications in the display tank.
- Feed small amounts 2–3 times daily during recovery.
For step-by-step quarantine, read our reef fish quarantine basics. It saves livestock and money. It also reduces stress in the display.
Most common reef tank problems improve with stable parameters and patience. Test, log, and adjust slowly. Build routines that prevent swings and pests. Your reef will reward steady care over quick fixes.
Sources: Randy Holmes-Farley, “Reef Aquarium Water Parameters”; Julian Sprung, “The Reef Aquarium” series; Eric Borneman, “Aquarium Corals.”
By
