What Eats Hair Algae? An Honest List From a Keys Collector

You pulled a fistful of green hair off the rock on Sunday. By Thursday it is back, an inch long and waving in the current. You bought ten snails. The snails are sitting on the glass, ignoring it. This is the honest list of what actually eats hair algae, what only looks busy, and what has to happen before any animal can win.

First, the honest part: no animal fixes nutrients

Hair algae is a fast-growing weed. It takes up nitrogen and phosphorus faster than corals do, so whenever those run high, it wins the race. The animals eat the algae. They do not eat the phosphate.

For scale: field research by Dr. Brian Lapointe on reefs in Jamaica and southeast Florida found that seaweed blooms are sustained at about 1.0 micromolar nitrogen and 0.1 micromolar phosphate. On a hobby test kit, that is roughly 0.06 ppm nitrate and 0.01 ppm phosphate. A mixed reef tank usually runs 5 to 15 ppm nitrate and 0.03 to 0.10 ppm phosphate, and that is where we recommend you keep it. At 5 ppm, your nitrate is about 80 times the level that feeds blooms on a wild reef.

So every reef tank is “rich” by wild-reef standards, and you cannot fix that without starving your corals. That is exactly why grazers matter more in a tank than on a clean reef. Nutrients alone can never get low enough to stop algae.

Why phosphate is usually the lever

Algae build tissue at roughly 16 parts nitrogen to 1 part phosphorus by molecule count, the Redfield ratio. A tank at 5 ppm nitrate and 0.03 ppm phosphate has about 255 nitrogen molecules for every phosphorus. Phosphorus runs out first on paper. That is why bringing phosphate into range usually slows hair algae faster than cutting nitrate.

Two test kit traps

• ppb phosphorus is not ppm phosphate. Some ultra-low checkers read phosphorus in parts per billion. Multiply that number by 3.066, then divide by 1,000, to get ppm phosphate. A reading of 30 ppb is 0.09 ppm phosphate, near the top of a mixed reef range.
• “Zero” phosphate with algae growing. A thick mat pulls phosphate out of the water as fast as it appears, so the test reads near zero while the algae grows. Pull the mats and retest. The jump you often see is the phosphate the algae was hiding.

Two more causes to rule out: light running longer than about 10 hours a day (we recommend 10 hours, about 8 at full power), and rock that is releasing stored phosphate back into the water. Check your RODI with a TDS meter too. It should read zero.

Make sure it is hair algae

“Hair algae” is a shape, not a species. Several unrelated things grow in strands, and different animals eat them.

What you see	Likely culprit	What eats it
Soft, fine, bright green strands that feel like wet cotton and pull off fairly easily	Derbesia, the most common green hair algae in reef tanks	Sea hares, urchins; snails and hermits on short regrowth
Feathery fronds with a central stem, like tiny ferns. Darker green, grips hard.	Bryopsis	Lettuce sea slugs. Most other grazers skip it.
Wiry, coarse strands like fishing line	Loose Cladophora or Chaetomorpha	Few grazers. Remove by hand.
Slimy red, brown or dark green sheet with bubbles	Cyanobacteria (not algae)	Almost nothing. Fix flow and nutrients.
Brown, snotty strings with bubbles, gone at night	Dinoflagellates	Nothing reliable. The fix is close to the opposite of hair algae.

One odd fact explains why hair algae is so hard to kill. Derbesia and bryopsis are siphonous algae. Each strand is one giant cell with many nuclei and no walls inside. Break a strand and the pieces can seal and keep growing. That is why scrubbing a rock with a brush inside the tank often makes things worse. You are planting it.

The grazers, ranked

This is every common cleanup crew animal ranked by how much hair algae it really removes. There is very little lab data on how much algae a snail eats. Anyone who gives you a precise number for that is guessing.

Animal	Long hair algae	Short regrowth	When it quits	Main risk
Sea hare	Yes, heavily	Yes	When the algae is gone, then it starves	Pump intakes, starvation
Pin cushion urchin	Yes	Yes	Never, if you feed it seaweed after	Moves frags, eats coralline
Reef urchin	Yes, more slowly	Yes	Never, if fed	Scrapes coralline, can dislodge frags
Algae-grazing snails	No	Yes, film and stubble	Starve if overstocked	Low
Scarlet reef hermit	Picks at short tufts	Yes	Switches to leftovers	Needs spare shells
Stocky cerith snail	No	Film on glass and sand	Rarely	Very low
Emerald crab	Inconsistent	Some	When fed pellets	May pick at corals when hungry
Lettuce sea slug	Bryopsis only	Bryopsis	When bryopsis is gone, then it starves	Pump intakes, starvation

Sea hares: the emergency crew

Nothing eats hair algae faster. A sea hare is a big, soft sea slug with two ear-like rhinophores on its head. Put one on a hairy rock and it plows through the mat.

There is no clean published number for the Florida species. The best data comes from a farming trial in Panama with the wedge sea hare, a Pacific relative. Over the first 26 days it gained 4.6 grams a day while it cropped the tank’s hair algae to almost nothing. Then the food ran out, and it lost 1.7 grams a day for 20 straight days. That is the whole story of a sea hare: it eats a mountain, then it starves.

Treat one as a two to four week job with an end date. Cover every pump and powerhead intake with foam before it goes in. A sea hare pulled into an impeller is a dead sea hare and a tank full of purple ink. Plan where it goes next: a friend’s tank with algae, a refugium with macroalgae, or back to the store. Sea hares are seasonal for us. When we have them, they are on the Sea Hare listing.

Urchins: the strongest permanent grazers

An urchin grazes with a five-toothed jaw called Aristotle’s lantern and scrapes rock clean down to the surface. It is the closest thing a reef tank has to the urchins that used to mow Caribbean reefs.

Pin cushion urchin (Lytechinus variegatus). The strongest hair algae grazer we collect. Our listing calls it a heavy grazer on hair algae, film algae and coralline. In a field study, one urchin ate about 0.6 grams dry weight of turtle grass a day, and dry weight means the wet pile is several times bigger. A 2012 study in St. Joseph Bay, Florida, found something more useful for reefers: pin cushions barely touched clean seagrass blades and went straight for blades coated in fine algae fuzz. Fine filamentous fuzz is what hair algae is. They also pick up shells and rubble and hold them on their spines, partly as sunscreen. In your tank, that includes frag plugs. Glue frags down first. Read more in our pincushion urchin care guide.

Reef urchin (Echinometra viridis). Smaller and tidier, with short spines. It hides by day and grazes film, hair algae and detritus at night. Slower than a pin cushion, but it does not carry things, so it is our pick for tanks full of small frags. See the reef urchin care guide.

Tuxedo urchin. Also a strong turf grazer, and it carries frags and shells like a pin cushion. It is an Indo-Pacific urchin, not a Keys animal, but it works. See the Tuxedo Urchin.

Once the algae is gone, clip a small piece of dried seaweed to the rock two or three times a week. A hungry urchin strips a tank clean and then starves.

Lettuce sea slugs: for bryopsis

The lettuce sea slug is a sap-sucker, not a sea hare. The research supports it on bryopsis. Adults in the Keys live on bryopsis and related green algae, and only newly hatched slugs in a lab study ate the soft green hair algae Derbesia. Buy one when your problem is the feathery, fern-like algae that nothing else touches. Read the lettuce sea slug care guide before you do.

Snails and hermits: steady pressure, not a rescue

Algae-grazing snails graze film, diatoms and fine short algae. They do not mow down long hair algae. Put them on a rock with a 2-inch mat and they work around it. Where they earn their keep is the week after you pull the mats. Hair algae grows back from the base, and snails keep that stubble short before it matters. If a snail flips onto its back on the sand and cannot right itself, turn it over.

Stocky ceriths are a different job. They graze glass and the sand surface, not hair. See the cerith snail care guide.

Hermit crabs are pickers. They pull at short tufts with their claws and will not clear a rock covered in long strands. If you want algae work without snail trouble, pick the scarlet reef hermit. It is more interested in algae and leftovers than in fighting. Whatever hermits you keep, add empty shells a size or two bigger than the ones they wear. A hermit that needs a bigger shell will take one off a live snail.

Emerald crabs: better for bubble algae

Emerald crabs are sold everywhere as algae eaters. The best study on them looked at bubble algae, not hair algae, and found that the crabs chose pellets 77 percent of the time when both were offered. On hair algae, results from reefers are all over the map. An emerald crab still earns a place in many crews because it works crevices at night that nothing else reaches. Just do not buy it as your hair algae answer. See the emerald crab care guide.

What does not eat hair algae

• Nassarius snails. Scavengers that live under the sand. Great sand crew, no hair algae value. See the nassarius guide.
• Fighting conchs. They work the top of the sand and never climb rock. See the fighting conch guide.
• Peppermint and coral banded shrimp. Meat eaters. One popular list says otherwise. It is wrong.
• Copepods. They graze microscopic films. Nobody has shown they control established hair algae.
• UV sterilizers. UV kills what floats through it. Hair algae is stuck to the rock.

How many to buy, and in what order

Size the permanent crew to the tank you will have in two months, not the outbreak you have today. A crew sized for the worst day starves once the algae is gone, and dead snails feed the next outbreak. Our cleanup crew calculator sizes an everyday crew from your volume, tank age and sand bed. The Complete Clean Up Crew, Florida Keys, by Tank Size packs those numbers in one box.

Buy in two rounds. Put in about two thirds of the permanent crew first and the rest three weeks later if the algae is still growing. Add a sea hare for heavy mats, or a lettuce slug for confirmed bryopsis, as temporary help with a plan for where each goes next.

A 30-day plan

When	What to do
Day 0	Confirm it is hair algae, not cyano or dinoflagellates. Test nitrate and phosphate (convert ppb phosphorus). Check RODI with a TDS meter. Photograph the three worst spots.
Days 1 to 3	Pinch clumps off at the base and siphon them out. Scrub heavy rocks in a bucket of tank water, never in the display. Change 10 to 15 percent of the water. Set the photoperiod to 10 hours. Cut feeding to what fish eat in a minute or two.
Days 4 to 7	Add the first round of crew. Cover intakes and add a sea hare if the mats were heavy. Start bringing phosphate down slowly if it is high for your tank.
Days 8 to 14	Test twice. Expect phosphate to rise as algae comes out. Pinch anything over half an inch. Re-glue anything the urchin moved.
Days 15 to 21	Add the rest of the crew if algae is still spreading. Plan the sea hare’s exit before it starts shrinking. Another 10 to 15 percent water change.
Days 22 to 30	Move the sea hare out. Start clipping dried seaweed for the urchin and snails. Retake your day 0 photos.

Success on day 30 looks like this: no strands longer than about half an inch, nitrate and phosphate steady in range, and a crew that looks busy, not hungry. If algae is still growing with good numbers, recheck the ID.

Acclimation

Urchins, slugs and crabs are sensitive to fast salinity changes. Turn the lights off and float the sealed bag for 15 minutes. Open it into a clean container and drip tank water in slowly for 45 to 60 minutes. Urchins should get the slow end of that range. Keep them in water when you move them and throw the bag water away. Never use copper in a tank with invertebrates.

FAQ

The Underwater Gardener is a reef aquarium store in Key West, Florida. Our urchins, sea hares, lettuce sea slugs, hermits and snails are collected by our team in the Florida Keys. Livestock ships UPS Next Day Air on Tuesdays and Wednesdays.