Green Hair Algae in a Reef Tank: Causes, Removal & What Eats It

Green hair algae in a reef tank is a fast-growing filamentous algae fed by excess phosphate and nitrate, long photoperiods and poor export. To beat it, pull it out by hand, cut nutrient inputs, boost export with water changes, skimming and GFO, then let grazers such as Turbo snails, tangs and lawnmower blennies keep regrowth short.

This guide covers how to tell green hair algae from Bryopsis, what causes it, a removal plan that works, the best hair algae eaters for a reef tank, and the treatments to use when nothing else works.

Updated October 2026 by the team at Natural Seawater Supplier, a Croydon, London-based supplier of live natural seawater for marine aquariums, delivering across the UK 7 days a week.

Key takeaways

• Green hair algae (often Derbesia or Cladophora species) grows when phosphate sits above about 0.1 ppm and nitrate above about 20 ppm in a reef, especially under long photoperiods.
• Bryopsis looks similar but has feathery, fern-like side branches; it is tougher, rarely eaten and usually needs a magnesium or fluconazole treatment.
• Manual removal plus nutrient control fixes most outbreaks in four to eight weeks; grazers keep it from coming back.
• The best green hair algae eaters for saltwater reef tanks are Turbo and Mexican turbo snails, sea hares, lawnmower blennies, yellow and sailfin tangs and foxface rabbitfish.

Is it green hair algae or Bryopsis?

Not sure? Our guide to the types of algae in a saltwater tank has a full identification table, including film algae, turf algae and bubble algae.

What causes green hair algae in a reef tank?

High nitrate and phosphate

Hair algae is a nutrient sponge. Keep nitrate under about 20 ppm (ideally 2–10 ppm) and phosphate under 0.1 ppm (ideally 0.02–0.08 ppm). Note that the algae absorbs nutrients as fast as they appear, so a low test reading during an outbreak can be misleading.

Overfeeding and frozen food juice

Pellets, flakes and frozen foods are the main nutrient input in most tanks. Thaw frozen food in a little tank water, strain it through a net and feed only what is eaten in two minutes. Our guide on how much and how often to feed marine fish gives practical portions.

Lighting that runs too long

Run reef lights for no more than about 8–10 hours at full intensity, and less on a fish-only system. If you keep demanding SPS corals, find the balance where they thrive without the lights running longer or brighter than necessary. See our reef tank lighting schedule.

Tap water, old rock and dirty media

Tap water top-off, phosphate leaching from old rock or dead live rock, and clogged filter sponges or bio balls all add nutrients. Hair algae can also arrive as fragments on new rock or frags.

How to remove green hair algae: step by step

1. Pull it out by hand. Twist clumps off the rock with a toothbrush or tweezers, ideally just before a water change so loose strands go out with the siphon. Do not scrub rock in the display without catching the fragments.
2. Siphon and change water. Change 15–20% of the water weekly during an outbreak, then 10–20% every one to two weeks; see how often to change water in a saltwater aquarium.
3. Cut inputs. Reduce feeding, rinse frozen food and top up only with RODI water.
4. Boost export. Tune the skimmer, replace filter socks every 3–4 days, run GFO or another phosphate remover if phosphate stays above 0.1 ppm, and grow chaeto in a refugium. Our nutrient export guide explains each option.
5. Reduce the photoperiod. Trim an hour or two from the peak, and block sunlight from windows.
6. Add grazers. Once the algae is trimmed short, add Turbo snails or a suitable fish. Grazers prefer short, fresh growth over long, tough strands.
7. Repeat weekly. Expect improvement in two to four weeks and a clean tank within about eight.

Removing rocks for scrubbing in a bucket of old tank water is an option for small rocks, but avoid freshwater dips on live rock, which kill beneficial life and release more nutrients.

What eats hair algae in a reef tank?

For the full list with reef-safety ratings and stocking ratios, see our guide to the best reef-safe algae eaters.

How to get rid of Bryopsis

Bryopsis needs the same nutrient work as hair algae, but most hobbyists also need a treatment. As of 2026 two approaches are widely used:

• Magnesium method (Kent Marine Tech M). Raise magnesium gradually to about 1,600–1,700 ppm, no faster than about 100 ppm per day, and hold it there for several weeks. It only seems to work with Kent Tech M specifically, results are not guaranteed, and you must watch alkalinity and sensitive invertebrates.
• Fluconazole. An antifungal that kills Bryopsis and often green hair algae. The common hobby protocol is a single dose of about 20 mg per US gallon (around 5 mg per litre) held for about 14 days, with a water change beforehand, carbon removed and the skimmer run dry. Ready-made products such as Maxspect Green Control have their own label dose. Expect a nutrient spike as the algae dies, and note that fluconazole will also harm chaeto and other green macroalgae.

Bulk Reef Supply’s guide to beating green hair algae is a good second reference on export methods and treatments.

How to stop hair algae coming back

Keep nitrate and phosphate low and stable, maintain the clean-up crew, quarantine new frags and test weekly until things settle. The water you use for changes should be cleaner than your tank water. Our natural seawater typically tests at nitrate 0–0.9 ppm, phosphate 0–0.05 ppm and silicate 0–0.1, so each change lowers nutrients rather than adding them. Try 25 litres free if you are a new customer, and read our complete guide to getting rid of algae in a saltwater tank for other algae types.

Frequently asked questions

What eats green hair algae in a saltwater reef tank?

Turbo and Mexican turbo snails, Trochus snails, sea hares, lawnmower blennies, yellow and sailfin tangs and foxface rabbitfish all eat green hair algae. They work best on short regrowth after you have pulled the long strands by hand.

What causes green hair algae in a reef tank?

Green hair algae is caused by excess phosphate and nitrate, overfeeding, long photoperiods and tap water top-off. It can also arrive as fragments on new live rock or coral frags.

How do I get rid of hair algae fast?

Pull out as much as you can by hand, siphon loose strands during a 15 to 20 percent water change and cut the photoperiod to about 8 hours. Add GFO if phosphate is above 0.1 ppm, then add Turbo snails to graze the regrowth.

Is Bryopsis the same as green hair algae?

No. Bryopsis has feathery, fern-like branches and is rarely eaten by grazers, while green hair algae has long unbranched strands that snails and fish eat readily. Bryopsis usually needs a magnesium or fluconazole treatment as well as nutrient control.

Will a lawnmower blenny eat hair algae?

Yes, lawnmower blennies graze hair and film algae from rock all day. Keep one per tank of at least 200 litres and offer nori once the algae is gone, or it may start nipping clams and LPS corals.

Bottom line

Green hair algae in a reef tank is a nutrient and light problem. Confirm it is not Bryopsis, pull it out by hand, lower phosphate and nitrate, shorten the photoperiod and let grazers keep regrowth in check. Most tanks are clear within two months.

Sources

External sources cited in this article (checked October 2026):

• Guide to beating green hair algae (Bulk Reef Supply)