Reef tanks can look perfect one day and stressed the next. Most problems come from small shifts in chemistry, flow, or maintenance timing. This guide helps you diagnose issues fast and fix them with clear steps.
Start with the basics: test, observe, and confirm
When something looks wrong, slow down and gather data. Test salinity, temperature, and pH first. These drive most reef stability. Aim for 35 ppt salinity, 77–79°F, and pH 8.0–8.3.
Next, test alkalinity, calcium, and magnesium. Keep alkalinity 8–9 dKH for mixed reefs. Keep calcium 400–450 ppm and magnesium 1250–1350 ppm. Write results in a log to spot trends.
Use your eyes like a test kit. Coral tissue recession often starts at the base. Fish flashing can signal irritation or parasites. Brown film on sand often points to nutrients and light balance.
Confirm equipment is working as expected. Check heater accuracy with a second thermometer. Verify ATO operation and reservoir salinity creep. Review reef tank water parameters if you need targets.
- Calibrate your refractometer with 35 ppt solution each month.
- Test alkalinity at the same time daily for one week.
- Clean powerhead intakes to restore flow and oxygen exchange.
Common reef problems and fast, safe fixes
Cloudy water often comes from bacterial blooms or sand storms. Check for overfeeding and clogged filtration first. Run fresh carbon for 48 hours. Increase surface agitation and aim for strong gas exchange.
Algae outbreaks usually follow excess nutrients or unstable export. Target nitrate 5–15 ppm and phosphate 0.03–0.10 ppm. If nitrate is zero, corals can pale. Feed a bit more or reduce aggressive export.
If corals are browning, check light and phosphate. High phosphate can dull colors. Try small changes only. Reduce photo period by 30–60 minutes per week. Avoid big PAR jumps after bulb or LED changes.
If corals are bleaching, suspect light shock, heat, or alkalinity swings. Keep alkalinity changes under 0.5 dKH per day. Lower peak intensity by 10% and ramp back over two weeks. Review reef lighting basics for safe acclimation.
- Do a 10% water change if you cannot identify the issue.
- Match new water to within 0.001 SG and 1°F of the tank.
- Replace old test reagents if results seem inconsistent.
Stability systems: dosing, nutrients, and quarantine habits
Many “mystery” issues are dosing problems. Dose alkalinity in small daily amounts. Split dosing into two or more events. Use a dosing pump if daily demand exceeds 0.5 dKH.
Keep nutrients steady, not ultra-low. Skimmers, refugiums, and socks must match feeding. If phosphate is 0.00 on a checker, corals can stall. Reduce GFO or media and feed more consistently.
Fish disease can look like water quality trouble. Quarantine new fish for 14–30 days when possible. Watch for spots, heavy breathing, and frayed fins. Use a separate observation tank and stable salinity.
Create a weekly routine you can repeat. Clean the skimmer cup twice per week. Change or rinse filter socks every 2–3 days. Use reef tank maintenance checklist to stay consistent.
- Keep temperature swing under 1°F per day.
- Keep salinity swing under 0.5 ppt per day.
- Change one thing at a time, then wait seven days.
Reef tank troubleshooting works best with calm, repeatable steps. Test key parameters, confirm equipment, and make small corrections. With stable targets and a simple routine, most tanks recover quickly.
Sources: Randy Holmes-Farley, “Reef Aquarium Water Parameters” (Reefkeeping Magazine); Julian Sprung, The Reef Aquarium series; Eric Borneman, Aquarium Corals.
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