Green Hair Algae: Causes and How to Get Rid of It

"Green hair algae" isn't one species. Reef tank keepers use it as a catch-all for a handful of fast-growing filamentous algae, most often Cladophora or Rhizoclonium, that show up as green tufts on rock and glass. It's worth knowing this isn't the same organism as Chaetomorpha, the "chaeto" hobbyists deliberately grow in a refugium for nutrient export. Same general look, very different role in your tank.

Is it really GHA, or is it Bryopsis?

Check this before you start treatment, because the wrong protocol wastes weeks. Bryopsis looks similar at a glance but grows stiffer, more feather-like, and noticeably larger than true GHA. The simplest field test is watching your cleanup crew: most reef-safe grazers eat true GHA readily and largely ignore Bryopsis. If your snails and blennies won't touch it, assume Bryopsis. The manual and biological methods below work well on true GHA but tend to fail on Bryopsis, which usually needs a targeted chemical treatment like fluconazole instead - Blue Life USA's Flux Rx is the commonly used product, and Reef2Reef's long-running "Bryopsis Cure" thread has the community-tested dosing and duration protocol (roughly 2 weeks for Bryopsis, longer for GHA/turf algae).

What actually causes it

Excess phosphate and nitrate from overfeeding, overstocking, or decaying organics is the primary driver, and GHA specifically favors bare rock that's still leaching phosphate from prior die-off. A second, often-overlooked cause is silicate contamination from tap water or an exhausted RODI resin or membrane, which lets silicates and phosphates pass straight into your tank.

GHA doesn't spontaneously appear either. It typically enters on live rock, live sand, coral frags, or new snails as a hitchhiker. From there it establishes itself if nutrients are available and your cleanup crew isn't large enough to graze it down early. Lighting spectrum and low flow get blamed a lot in hobby discussion, but the evidence for either as a primary cause is weak. Treat them as secondary factors that can tip an already nutrient-rich tank over the edge, not root causes on their own.

Target nutrient ranges (and where sources disagree)

Reef keepers don't fully agree on exact numbers here, so use these as a range rather than a single target:

The two don't agree exactly, especially on nitrate, but they agree on the principle that matters most: low but detectable beats zero. More on why zero backfires below.

How to actually remove it

1. Manually remove what's there. Pinch clumps off between your fingers and rinse them off into a cup rather than letting them float loose in the tank - Melev's Reef covers this technique in detail. Skip the scrub brush: per AquariumStoreDepot, scrubbing just sets fragments drifting to reattach somewhere else in the tank instead of removing them.
2. Build up your cleanup crew. Astraea and Mexican Turbo snails graze algae off rock surfaces, though Turbos can't process long strands on their own. Lawnmower blennies and tangs are commonly used too, though individual appetite varies more than retailers usually admit. Check any new addition against the Livestock Compatibility Checker before you buy.
3. Control the nutrients feeding it. GFO (granular ferric oxide) adsorbs phosphate only and does nothing for nitrate. Biopellets work differently: they feed bacteria that consume nitrate and phosphate together, but that bacterial process needs some phosphate present to run at all. Push phosphate to true zero and biopellets stop working on nitrate too.

Does the blackout method work?

A multi-day blackout can knock algae back by cutting off the light it needs to photosynthesize. Three days is the duration most commonly used in reef tanks specifically. Longer blackout windows you'll see cited elsewhere are typically freshwater planted-tank advice, not reef tank advice. But ReefBum's own testing found a blackout alone won't permanently solve anything. Unless you fix the actual nutrient source at the same time, the algae grows right back within a couple of weeks. Treat blackout as a supporting tactic, not the fix itself.

Mistakes that make this take longer

Removing algae without addressing nutrients is the single most common mistake. It comes back, often within days, because the thing feeding it never went away. Running a blackout without fixing nutrients fails for the same reason.

The other common mistake runs the opposite direction: overcorrecting nutrients all the way to zero. Undetectable phosphate and nitrate are widely linked to dinoflagellate outbreaks, which are considerably harder to deal with than hair algae. Aim for the low-but-detectable ranges above, not zero.

One more myth worth killing directly: a UV sterilizer won't touch hair algae. UV only affects free-floating organisms passing through the water column, and hair algae is attached to rock and glass, not floating.

FAQ

Will a UV sterilizer get rid of green hair algae?
No. A UV sterilizer only affects free-floating organisms passing through the water column. It has no effect on algae already attached to rock or glass, which is where hair algae actually grows.

Should I just remove all the nitrate and phosphate to stop algae for good?
No, and this is a common overcorrection. Driving nutrients to true zero is widely linked to dinoflagellate outbreaks, which are harder to deal with than hair algae. Aim for low but detectable, not undetectable.