Green hair algae in a saltwater tank: what eats it, and what actually stops it
It started as a few green tufts on one rock. Three weeks later it is a lawn, the snails are sitting on top of it looking bored, and your search history reads "hair algae saltwater tank" followed by "what eats hair algae".
The honest answer before the list: several animals eat hair algae, and none of them stop it. Green hair algae is a nutrient and light problem, phosphate above all, and a grazer only turns algae into fish waste, which is phosphate again. A clean-up crew can hold the line once growth is slow. It cannot get you there. Grazers first, because that is what you searched for, then what actually works.
First, check it is green hair algae
Green hair algae proper (usually Derbesia, sometimes Cladophora) is soft, bright green and pulls off the rock in a mat between finger and thumb. This is what the rest of the post is about.
Bryopsis gets mistaken for it constantly. It has a stiff central stem with side branches like a feather, and it snaps rather than pulls. Almost nothing eats it. The fix is fluconazole (sold as Reef Flux) at about 5 mg per litre (20 mg per gallon) for 14 days with carbon out, not a grazer.
What eats hair algae, and what each one will not do
| Grazer | Eats | Will not eat | The catch |
|---|---|---|---|
| Sea hare (Dolabella auricularia) | Hair algae, in bulk | Bryopsis, turf | Starves once the algae is gone; inks when stressed; finds pump intakes |
| Turbo snail (Turbo fluctuosa) | Hair algae, film | Bryopsis | Bulldozes frags; dies quietly when food runs out |
| Foxface (Siganus vulpinus) | Hair algae, including longer strands | Thick mats | May sample LPS and zoanthids; venomous spines; needs a 1.2 metre (4 ft) tank |
| Zebrasoma tangs (yellow, purple, sailfin, scopas) | Short new growth, all day | Thick mats | Needs 1.2 metres (4 ft) or more; aggressive to other tangs |
| Tuxedo urchin (Mespilia globulus) | Hair algae, coralline | Bryopsis | Carries frags on its back; hates salinity swings |
| Emerald crab (Mithraculus sculptus) | Bubble algae, hair algae when hungry | Much, once well fed | Large ones pick at corals and snails |
Tangs are not interchangeable. The Zebrasoma tangs are the hair algae grazers. Bristletooth tangs (kole, tomini and the other Ctenochaetus) are built for film and detritus and will not touch a mat of hair algae. Neither will a lawnmower blenny, in spite of the name.
Nothing eats a mature mat well. Once it is 2 cm long and packed with detritus, grazers work around the edges. Pull it out by hand first, then let them manage the regrowth.
Every one of these animals is temporary or a liability. A sea hare will strip a 200 litre (50 gallon) tank in three to four weeks and then starve, so arrange with your shop to take it back. Drip acclimatise urchins and sea hares slowly, with bag and tank within roughly 0.001 SG (about 1.3 ppt), because neither forgives a salinity jump.
Grazers recycle, they do not export. An algae mat is stored phosphate and nitrogen. A tang eats it and returns most of it to the water as waste within a day. Only your hands, the skimmer, media, water changes and harvested macroalgae actually export.
Why hair algae is a nutrient and light story
Hair algae needs light, nitrogen and phosphorus. Phosphate is the lever, because hair algae can take nitrogen as ammonium straight out of fish waste before it ever shows on a nitrate kit.
The ranges most reef keepers run are nitrate 2 to 10 ppm and phosphate 0.03 to 0.10 ppm. Tanks sitting at 0.2 to 0.5 ppm phosphate and 25 ppm plus nitrate grow hair algae almost without exception. Tanks held inside those bands grow a little film on the glass and not much else, once the rock has stopped leaching. The full target chart, and which number to fix first, is in reef tank parameters.
"My nutrients read zero and I still have hair algae"
The most common message in any hair algae reef tank thread, and the reading is usually true. The water column is not the reservoir. The algae is.
Phosphate enters with food and top-off water every day. A large mat takes it up as fast as it arrives, so the water reads 0.00 and the algae keeps growing. Zero is not evidence of low input. It means something is eating the input before your kit sees it, and that something is attached to your rock. Nitrate is the same: algae prefer the ammonium fish produce, and the nitrate kit never sees it.
Use a kit that can read the range: colour-matching kits are honest to about 0.03 ppm at best, while a Hanna ULR checker reads ppb of phosphorus, and ppb times 3.066 divided by 1000 is ppm phosphate, so 30 ppb is 0.09 ppm. And do not raise nutrients to "feed the corals" while the mat is still there. The corals are pale because the algae is winning, not because the tank is starved. Pull the algae out and the readings will start to mean something.
Hair algae and phosphate: where it is actually coming from
Before you buy media, find the source. The hair algae phosphate link is real, but pulling phosphate out of the water while something keeps adding it is a treadmill.
Rock and sand. Aragonite binds phosphate to its surface, and dry rock that spent years as a dead reef can hold a lot of it, as can a sand bed full of detritus. Lower the water column with GFO and the rock gives it back over a few days, which is why "I ran GFO, it hit zero, it came back" is so common. Test it: a fist-sized piece of rock in a bucket of freshly mixed saltwater that reads zero phosphate, 24 hours, then test the bucket. At 0.05 ppm or more the rock is leaching, and you plan in months rather than weeks.
Feeding. The most common cause. Frozen food thawed in its own juice carries a slug of dissolved phosphate before a fish touches it. Thaw, strain, feed the solids only, and feed what disappears in a minute or two. Fish twice a day, coral food and amino acids on top is more input than a 100 litre (25 gallon) tank can export.
Tap water. Many water utilities add orthophosphate as a corrosion inhibitor, often at around 1 ppm. Tap top-off concentrates it, because the water evaporates and the phosphate stays. Use RO/DI that reads 0 TDS, and change the DI resin when the output reads 3.
Old bulbs. T5 and metal halide lamps shift toward red and yellow as they age and lose a good part of their output by 12 months, and that shift favours algae over coral. Replace T5s every 9 to 12 months, one or two a week, because a fresh set can double the PAR and bleach corals. LEDs do not shift the same way, but a white channel at 100 percent for 12 hours is a hair algae growing programme; ten hours of full spectrum is plenty.
The salt itself. Some mixes carry measurable phosphate, so test a fresh batch before it goes near the tank. Does your reef salt mix change your parameters covers what varies between brands.
How to get rid of hair algae in a reef tank: the export toolkit
If you searched "how to get rid of hair algae reef tank", this is the section. Use these together. Any single one is too slow.
1. Manual removal, weekly, before anything else. Pull it off the rock with your fingers while a siphon hose runs beside your hand, so the fragments leave the tank instead of reseeding it. Rocks you can lift come out and get a stiff toothbrush in a bucket of tank water. For bare rock with nothing living on it, a five minute dip in 3 percent hydrogen peroxide and a rinse kills what the brush missed; never dip a rock with coral or invertebrates on it, and never pour peroxide into the display. Every clump you remove is phosphate and nitrogen gone for good. It is the most effective export here, and free.
2. GFO. In a reactor tumbling gently, or a media bag in strong flow. Start at half the label dose, about 1 tablespoon per 30 litres (8 gallons) for standard GFO, and halve that again if phosphate is above 0.3 ppm, because crashing phosphate to zero in a day stresses corals, and SPS in particular go pale or lose tissue. Test after three or four days. Replace the media when phosphate starts to climb again, typically every four to six weeks, and only then work up toward the full label dose of 1 to 2 tablespoons per 15 litres (4 gallons).
3. Lanthanum chloride, with care. It binds phosphate into a fine precipitate that must be caught. Dilute it heavily in RO/DI water, drip it slowly into the overflow or a reactor ahead of a 10 micron filter sock, and never dose it into the display, because the precipitate clogs fish gills, and tangs in particular have died from it even with a sock in place. Bring phosphate down over a week or two, not an afternoon, and stop if any fish breathe hard.
4. Flow and detritus. Detritus is fertiliser sitting on the rock, and hair algae grows wherever it settles. Aim a powerhead at the dead spot behind the rockwork, baste the rocks before a water change, and change filter socks every two or three days.
5. A refugium that is big enough. Chaetomorpha under a proper grow light competes for the same phosphate and nitrate, and every handful you throw away is real export. A 10 litre (2.5 gallon) chamber under a desk lamp will not out-compete a 400 litre (100 gallon) display.
6. Carbon dosing, if nitrate is also high. Vodka, vinegar or a commercial blend grows bacteria that take up nitrate and some phosphate, and the skimmer exports them. Start at a quarter of the bottle dose and increase slowly over weeks, because an overdose produces a bacterial bloom that clouds the water and can drop oxygen overnight. The full method is in how to lower nitrates in a reef tank.
Water changes of 10 to 20 percent weekly are a good habit, but they only remove the dissolved fraction: a 20 percent change at 0.2 ppm takes you to 0.16 ppm, and the rock puts it back by the weekend.
What to expect, and in what order
If you do all of it:
- Week 1. Manual removal, RO/DI, feeding fixed, GFO at the low dose.
- Weeks 2 to 4. Phosphate readings appear as the algae mass shrinks. Keep pulling weekly.
- Weeks 4 to 8. Regrowth is short and patchy. Now add the grazers: a Zebrasoma tang or foxface for 1.2 metres (4 ft) or more, a tuxedo urchin and a few turbos for smaller tanks. They keep new growth down; they could not have cleared the mat.
- Weeks 8 to 12. Phosphate holds inside 0.03 to 0.10 ppm without rebounding after a GFO change, which means the rock has stopped giving. Back GFO off or stop, and send the sea hare home.
Rock that leached heavily can drag the last stage out to six months. That is not a failure, it is a bucket emptying at the rate it empties.
Log phosphate and nitrate weekly, on the same day
None of this is visible without numbers. Phosphate and nitrate once a week, same day and hour, same kit, with the date of every GFO change and rock scrub next to them. That is how you tell "GFO exhausted" from "rock still leaching": the first climbs slowly from week four, the second bounces within days of every media change. The cadence for the other parameters is in reef tank water testing schedule.
Get the spreadsheet
Make a copy in Google Sheets (needs a Google account)
Download the Excel version (no account needed)Free, no email, no attribution. Readings turn green inside your target band and red outside it, and the charts draw your target band as dashed lines.
A hair algae fight is eight to twelve weeks of phosphate readings, media change dates and rock scrubs that only make sense laid side by side, and a notebook never shows you the rebound after a GFO change. Recif is the reef companion app we built to handle this: it logs every test, charts phosphate and nitrate against your own target ranges so a creeping climb is obvious before the rock turns green again, works out consumption trends, reminds you when a parameter has gone too long untested, and rolls the whole tank into one Stability Score. It is available on both the App Store and Google Play.