Algae is normal in reef tanks. It becomes a problem when it smothers corals and ruins flow. This guide shows practical steps that work in real systems.
Section 1: Diagnose the algae and stabilize nutrients
Start by naming the algae. Green film often points to light and nutrients. Hair algae suggests excess phosphate and trapped detritus. Cyanobacteria is usually a flow and nutrient imbalance.
Test and log key numbers for two weeks. Aim for nitrate at 2–15 ppm. Keep phosphate at 0.03–0.10 ppm. Keep salinity at 1.025 and temperature near 78°F.
Avoid chasing “zero nutrients.” Corals need some nitrate and phosphate. Ultra-low levels can cause pale corals and dinoflagellates. Stability beats perfection every time.
Use consistent export methods. Change 10–15% water weekly for four weeks. Clean mechanical filters every 2–3 days. Empty the skimmer cup twice weekly and scrub the neck.
- Quick baseline targets: NO3 2–15 ppm, PO4 0.03–0.10 ppm, alkalinity 8–9 dKH
- Daily habit: remove visible detritus from bare spots with a turkey baster
- Weekly habit: test NO3 and PO4 before water changes
If algae spikes after feeding, measure your input. Feed fish what they finish in 30–60 seconds. Rinse frozen food in RO/DI water. Use a feeding ring to reduce drift into rocks.
For deeper nutrient control, consider controlled media. Start GFO at 25–50% of the recommended dose. Test phosphate every 2–3 days at first. Rapid drops can stress LPS corals.
For more on stability, review reef tank water parameters. If you suspect dinos, see dinos vs cyano identification.
Section 2: Manual removal, clean-up crews, and flow fixes
Manual removal is the fastest win. Pull hair algae during a water change. Twist it around a toothbrush. Siphon at the same time to remove loose strands.
Scrape glass and blow rocks before filtration. Use a powerhead or baster. Run filter socks or floss for 24 hours. Then replace them to avoid nutrient release.
Build a clean-up crew that matches the algae type. For film algae, add trochus snails at one per 3–5 gallons. For hair algae, add a few turbo snails in larger tanks. Add hermits carefully and provide spare shells.
Flow matters more than most people think. Dead spots trap detritus and feed algae. Aim for 20–40x tank turnover per hour. Point pumps to create random, crossing currents.
- Manual routine: scrape, blow, siphon, then swap filter media
- Flow check: drop a pinch of food and watch where it settles
- Clean-up crew tip: acclimate snails slowly over 45–60 minutes
Watch for common mistakes. Too many snails can starve once algae drops. Over-cleaning rock can strip beneficial biofilm. Avoid “blackout” fixes unless you know the algae type.
If you fight cyano, increase surface agitation and export. Reduce trapped organics in sand. Vacuum small sections each week. Consider a temporary 2–3 day light reduction, not total darkness.
For equipment tuning, see protein skimmer tuning. It helps remove dissolved organics before algae uses them.
Section 3: Light, refugiums, and long-term prevention
Light drives algae growth. Reduce white channels first. Many reefs do well with 8–9 hours total photoperiod. Keep ramp times short, around 30–60 minutes. Avoid 12-hour full-intensity schedules.
Check PAR if you can. Many soft corals prefer 50–150 PAR. Many LPS prefer 75–200 PAR. High PAR with high nutrients often fuels hair algae. Lowering PAR by 10–20% can help.
A refugium can outcompete nuisance algae. Use chaetomorpha with strong flow and tumble if possible. Run the refugium light on a reverse cycle for 10–14 hours. Harvest a softball-sized clump every 1–2 weeks.
Long-term success comes from repeatable routines. Keep RO/DI at 0 TDS. Replace carbon monthly if you use it. Clean pumps and powerheads every 2–3 months to maintain flow.
- Prevention checklist: RO/DI 0 TDS, stable nutrients, consistent export
- Refugium rule: harvest often to keep growth fast
- Lighting rule: reduce whites before cutting blues
Troubleshoot with a simple scenario. If nitrate is 0 and phosphate is 0.20, reduce phosphate slowly. Feed slightly more or dose nitrate to 2 ppm. This prevents imbalance and helps corals compete.
If nitrate is 25 and phosphate is 0.02, reduce feeding and detritus. Increase water changes for a month. Consider a refugium or bacteria-based export. Keep phosphate from bottoming out.
Sources: Randy Holmes-Farley, “Phosphate and the Reef Aquarium”; Julian Sprung, “Algae: A Problem Solver Guide”; Eric Borneman, “Aquarium Corals”
Algae control is a process, not a single product. Identify the algae, then balance nutrients and flow. With steady habits, your reef will look cleaner each week.
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