Cyano in reef tank setups is common, even in mature systems. It can spread fast and smother sand, rock, and corals. The good news is you can beat it with a plan and steady habits.
What cyano is and why it shows up
Cyanobacteria is a photosynthetic bacteria that forms slimy mats. It often looks red, burgundy, or dark purple. It can also appear brown or green. It traps bubbles during the light cycle.
Cyano thrives when nutrients and organics build up in pockets. It also likes low flow zones on sand and behind rock. Many outbreaks start after a change in feeding, lighting, or filtration. A new salt mix can also shift balance.
Test results can confuse hobbyists. You may see nitrate at 0–2 ppm and phosphate at 0.00–0.03 ppm. Cyano can still grow because it consumes nutrients fast. It can also use dissolved organics your kits do not measure.
Start with a quick system check. Confirm salinity at 1.025–1.026 and temperature at 77–79°F. Keep alkalinity stable, like 8–9 dKH for most mixed reefs. Sudden swings stress corals and slow healthy competitors.
- Common triggers: overfeeding, dirty filter socks, weak skimming, and dead spots
- Risk changes: new lights, longer photoperiods, and disturbed sand beds
- Hidden sources: old RO/DI filters and exhausted carbon
For more baseline targets, review reef tank water parameters. It helps you spot the first imbalance.
Step-by-step control without nuking the tank
First, remove as much cyano as you can. Siphon mats during a water change. Use 1/2 inch tubing to lift sheets off sand. Aim for 10–20% water changes weekly until it slows.
Next, fix flow where cyano returns. Add a small wavemaker or redirect a nozzle. You want sand to ripple lightly, not blow bare. Target 20–40x total turnover in many reef tanks.
Then reduce excess inputs. Feed fish what they eat in 30–60 seconds. Rinse frozen food in RO/DI water. Clean your skimmer cup twice weekly. Replace filter floss every 2–3 days.
Balance nutrients instead of chasing zeros. Many tanks do well with nitrate 5–15 ppm and phosphate 0.05–0.10 ppm. If both read near zero, corals can pale and cyano can persist. Consider lighter export or a bit more feeding.
- Siphon daily for three days, then weekly until clear
- Shorten lights by 1–2 hours for two weeks
- Run fresh carbon and change it every 2–4 weeks
If you run a refugium, keep it stable and clean. Harvest macroalgae weekly. Remove detritus under the algae mass. For setup tips, see refugium setup guide.
Troubleshooting stubborn outbreaks and common mistakes
If cyano returns within days, look for trapped detritus. Check the back chambers, sump corners, and under rock ledges. Blow rocks with a turkey baster before water changes. Let the skimmer pull the cloud out.
Verify your source water. RO/DI should read 0 TDS. Replace sediment and carbon when pressure drops or chlorine breaks through. Change DI resin when TDS rises to 1–2. Bad water can fuel repeat blooms.
Avoid the biggest mistake: aggressive, sudden nutrient stripping. Heavy GFO, oversized carbon dosing, and huge water changes can crash phosphate. That can stall corals and favor cyano. Make changes in small steps over 7–14 days.
Antibiotic treatments can work, but treat them as a last resort. They can reduce beneficial bacteria and lower oxygen. If you use one, add strong aeration and watch pH. Remove carbon during treatment and resume it after.
- Watch at night for low oxygen and gasping fish
- Do not stir deep sand layers in one session
- Do not increase lights to “outgrow” cyano
Quarantine and stability also matter. A stressed tank blooms more often. Build a routine using weekly reef tank maintenance checklist.
Cyano in reef tank systems is beatable with steady removal and better flow. Keep nutrients in a healthy range and export waste consistently. With a few weeks of discipline, your sand and rock can stay clean.
Sources: Borneman, E. (2001) Aquarium Corals; Delbeek, J. & Sprung, J. (1994–2005) The Reef Aquarium Vol. 1–3; Fenner, R. (2003) The Conscientious Marine Aquarist.
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