Nuisance algae shows up in every reef tank at some point. It can smother corals and ruin flow. This guide walks you through practical, repeatable control steps.

Identify the algae and confirm the cause

Start with a quick ID before you “treat the tank.” Green hair algae forms long, soft strands. Bryopsis looks like fern fronds on a single stalk. Cyano is a slimy mat that traps bubbles. Diatoms look like brown dust on sand and glass.

Next, confirm your nutrient picture with numbers. Test nitrate and phosphate weekly for a month. Aim for nitrate at 5–15 ppm. Aim for phosphate at 0.03–0.10 ppm. Zero readings can still mean algae is consuming nutrients fast.

Also check the basics that drive algae growth. Keep salinity at 1.025–1.026 SG. Hold temperature at 77–79°F with less than 1°F swing. Keep alkalinity stable at 8–9 dKH. Big swings stress corals and favor algae.

Look for the hidden inputs. Overfeeding is the top trigger in mixed reefs. A clogged filter sock can act like a nutrient reactor. Old light schedules also matter. Many tanks do better with 8–9 hours of full intensity.

• Take photos weekly from the same angle for progress tracking.
• Test NO3 and PO4 on the same day each week.
• Inspect RO/DI with a TDS meter and change resin at 1–2 TDS.

Mechanical removal and nutrient export that actually works

Manual removal is the fastest way to reduce biomass. Pull rocks and scrub in saltwater if possible. Use a toothbrush for crevices. Siphon the loosened algae through a 200-micron sock. This prevents it from spreading.

Then improve export so it stays gone. Clean your skimmer cup twice per week. Wet skim during outbreaks for faster removal. Replace or rinse filter floss every 2–3 days. Detritus breaks down into nitrate and phosphate.

Water changes help most when paired with removal. Try 10–15% weekly for four weeks. Match salinity and temperature closely. If phosphate is high, use a small amount of GFO. Start at 25–50% of the label dose. Rapid drops can stress LPS.

Consider a refugium or algae reactor for long-term stability. Start with a baseball-sized clump of chaeto. Run the refugium light 10–14 hours opposite your display. Harvest half the mass every 1–2 weeks. This exports nutrients you can measure.

• Target feed fish and corals, then stop pumps for 5–10 minutes.
• Rinse frozen food in RO/DI to reduce phosphate-rich juice.
• Vacuum 25% of the sand bed each week to avoid nutrient spikes.

Biological control, light tuning, and safe “chemical” options

A clean-up crew works best after you reduce algae mass. Add grazers that match your algae type. For hair algae, use trochus and turbo snails. For film algae, add ceriths and nassarius for sand turnover. Add slowly to avoid starvation later.

Fish can help, but tank size matters. A lawnmower blenny needs steady grazing and mature rock. Many tangs need 75 gallons or more. Quarantine new fish for 14–30 days. Use your quarantine for reef fish routine to prevent ich setbacks.

Dial in lighting to limit fuel without starving corals. Reduce white channels first. Try 10–20% less white for two weeks. Keep blues stable for coral color. Replace old bulbs on schedule. Many T5 bulbs shift spectrum after 9–12 months.

For stubborn cases, use targeted tools with caution. Hydrogen peroxide spot treatment can help on rocks. Use 1–2 mL of 3% peroxide per 10 gallons as a daily maximum. Turn off pumps and apply with a syringe. For bryopsis, some tanks respond to raising magnesium to 1500–1600 ppm using a known product. Test daily and avoid sudden jumps.

Common mistakes slow progress. Do not chase “zero nutrients.” Corals can pale and stop growing. Do not add a huge clean-up crew at once. Do not change three variables in one week. Use your reef tank water parameters checklist to stay consistent. Track changes in a log.

• Set a feeding cap: 1–2 minutes of consumption per feeding.
• Keep phosphate above 0.03 ppm to avoid dinoflagellates.
• Clean powerheads monthly to maintain strong, even flow.

Nuisance algae control is a process, not a product. Remove biomass, then balance nutrients and export. Add the right grazers and tune lighting for stability. If you stay consistent, most tanks turn the corner in 4–8 weeks.

For deeper troubleshooting, compare your symptoms to our common reef tank problems guide. Keep testing, keep notes, and make one change at a time.

Sources: Randy Holmes-Farley, “Phosphate and the Reef Aquarium”; Julian Sprung, “Algae: A Problem Solver Guide”; Eric Borneman, “Aquarium Corals.”

Bookmark