Green hair algae (GHA complex)

Identification

Soft green filaments like tufts of hair or grass, from short fuzz to strands many centimeters long, forming loose mats on rock, plugs, back glass and equipment. Filaments are thin and unbranched or loosely tangled (Derbesia type), feel soft and slippery, and tear away in clumps more easily than wiry turf algae.

Often confused with

Bryopsis shows stiff, feathery fern fronds with clear branching and a rooted holdfast; GHA strands are plain hairs with no feather pattern. Turf algae (Cladophora) is short, coarse and wiry like a scouring pad and resists pulling. A microscope view separates coenocytic Derbesia/Bryopsis from cell-divided Cladophora types.

What it targets

Rockwork, frag plugs, coral skeletons and bases, back glass and sand edges. Long mats shade corals, irritate polyps at contact points, and collect detritus that feeds further growth.

How it spreads

Spores and fragments arrive on frags, plugs and rock, and fragments from careless in-tank scrubbing can settle elsewhere. Blooms are fueled by phosphate (including phosphate bound in dry rock that leaches over time), nitrate, available iron and strong light, most often in tanks in their first year.

Control (safest first)

Manual removal, repeated: pull mats out by hand, scrub removed rock with a brush, and get roots and bases out with a pick on stubborn patches. Grazers only keep short regrowth cropped, so hand removal leads. Nutrient export: lower and stabilize phosphate with water changes, skimming, phosphate media or a refugium or algae scrubber growing macroalgae elsewhere. Go steady rather than stripping nutrients to zero, which can invite dinoflagellates. Grazing fish sized to the tank: tangs (including sailfin and kole types) and foxface rabbitfish crop hair algae in larger systems, and lawnmower blennies work smaller tanks. Stocking must match tank size and the fish stays after the algae is gone. Clean-up crew for maintenance: turbo snails (including Mexican turbos), Trochus, Astrea, emerald crabs, hermits and pincushion urchins are all reported useful on short growth, and all are reported useless against long mature mats until the mats are pulled short first. Sea hare: a heavy-duty grazer that can clear heavy GHA fast. It can starve once the algae is gone, so keepers pass it to another tank or feed it algae sheets. See field notes for the disagreement about risk when one dies. Fluconazole products: Derbesia is reported sensitive to fluconazole in many cases, while Cladophora-type hair algae can be resistant at standard doses. One keeper reported complete clearance in a couple of weeks after grazers and manual work failed. Use the Bryopsis entry dosage guidance from the product label and treat this as a later step. Bottled bacteria/algaecide products such as Vibrant: some keepers report fast clearance, others report follow-on cyano after die-off. Rank below manual, nutrient and grazer work.

Natural predators

• Turbo snails→
• Trochus snails→
• Sea hare→
• Tangs
• Foxface rabbitfish→
• Lawnmower blenny→
• Emerald crabs→
• Pincushion urchin→

Field notes

In a Reef2Reef best-solutions thread, keepers converged on the same stack: manual removal, nutrients kept in check and a strong snail crew, with one warning not to overdo nutrient stripping and trade hair algae for dinoflagellates. Sea hare reports split: several keepers call it the fastest fix for heavy hair algae, one warns a dead hare releases toxins, and another replies that claim is dramatic, reporting hares lost to a stinging coral with no toxic fallout after prompt removal. A keeper who tried urchins, sea hares, snails and exhaustive manual removal for months suspected phosphate bound in dry rock, then reported a fluconazole product (FluxRX) eradicated the hair algae within a couple of weeks.