| What It Is | Nuisance green filamentous algae |
| Looks Like | Green fuzzy/stringy turf on rock & glass |
| Main Cause | Excess nitrate & phosphate, light |
| Difficulty | Moderate — a nutrient problem |
| Best Fixes | Nutrient export + cleanup crew + manual |
Overview
Green hair algae (GHA) is the classic “my tank is ugly” nuisance — soft green filaments that carpet rock, sand and glass. It isn’t really a pest you kill so much as a symptom you correct: hair algae grows because there’s surplus nitrate and phosphate plus light to use them. Fix the nutrient side and starve it out, help it along with grazers and manual removal, and it recedes. Skip the root cause and it comes back no matter how much you pull.
How to Identify It
Bright-to-dark green, soft, hair-like strands from a few millimeters to several centimeters long, often slippery and clinging in tufts. It anchors hard to rock. (Two relatives are tougher — Bryopsis looks feathery/fern-like, and Derbesia is finer; both resist grazers, and Bryopsis often needs an elevated-magnesium treatment or Fluconazole to beat. If normal cleanup crews won’t touch your “hair algae,” suspect Bryopsis.)
How to Get Rid of It
- Export nutrients — the real fix. Test nitrate and phosphate; bring them down with water changes, a good skimmer, feeding less, and phosphate-removing media (GFO) or carbon dosing. Starve the algae and it can’t hold on.
- Manual removal — pull/twist tufts out at every water change (do it with a toothbrush on rock), removing biomass so it can’t re-release nutrients as it dies.
- Cleanup crew & grazers — emerald crabs, turbo and other grazing snails, and herbivorous fish (a tang or a lawnmower blenny) keep new growth mowed while nutrients drop.
- Check your source water — RODI with zero TDS; tap or exhausted filters add the very phosphate and nitrate feeding the bloom.
Prevention
Feed deliberately, keep the skimmer working, run RODI water, and don’t overstock. A modest cleanup crew and stable low-ish nutrients (not zero — that brings other problems) keep GHA from ever getting a foothold. New tanks often go through an algae phase as they mature; stay consistent and it passes.
What Doesn’t Work
Pulling algae without fixing nutrients: it grows right back — you removed the leaf, not the cause. Chasing zero phosphate overnight: crashing nutrients too fast stresses corals and can trigger dinos or cyano. Blaming light alone: reducing photoperiod helps, but nutrients are the lever that matters most.