Hair Algae in a Reef Tank: Causes and How to Get Rid of It

Where it comes from

Bulk Reef Supply notes that hair algae usually arrives without anyone noticing, on live rock, live sand, coral frags or new snails. The spores can be almost invisible. Once it’s in, it grows wherever there’s light and spare nutrients.

Two things make an outbreak more likely: a new tank, where nutrients swing and few grazers are established, and a mature tank where feeding has crept up or filtration has slipped.

Step 1: Pull it out

Pinch off clumps by hand and siphon them out straight away. Bulk Reef Supply suggests narrow RO tubing for getting into tight spots, and warns against scrubbing with a brush, which spreads fragments.

Do a little every few days rather than one big session. Each clump you remove takes its nutrients out of the tank with it.

Step 2: Bring in grazers

A clean-up crew is the first line of defence. Bulk Reef Supply lists fish such as tangs (especially Zebrasoma species), rabbitfish and lawnmower blennies. It also lists invertebrates such as Mexican turbo snails, trochus and astraea snails and tuxedo urchins.

Choose animals that suit your tank size and other livestock. A tang needs a large tank, and a lawnmower blenny may ignore algae if it’s well fed. Check each one before buying.

Step 3: Starve it

Hair algae needs nitrate and phosphate. Bulk Reef Supply gives targets of 0.02–0.06 mg/L phosphate and 5–10 mg/L nitrate for tanks fighting it. Bring high levels down with the usual fixes: feed less, clean filter socks often and add an algae export if you need one. Our guides to high nitrate and phosphate go through them.

Bulk Reef Supply warns about one trap: a tank full of algae can test at zero nitrate and phosphate, because the algae uses them up as fast as they appear. A zero reading doesn’t mean the problem has gone. Keep removing algae and keep testing.

What doesn’t help

Stripping nutrients to zero can starve corals before it starves the algae. Red Sea’s algae management guide stresses gradual changes for this reason. Cutting the lights for long periods can work against your corals too.

Changing several things at once makes it hard to see what’s working. Pick your actions, put them on a schedule and give them a few weeks.

Keep at it

A realistic routine: remove algae every few days, test nitrate and phosphate twice a week and keep up your water changes. Log each test. The trend shows when nutrients are under control, usually before the algae visibly retreats.

How long does it take to get rid of hair algae?

Usually weeks, sometimes a couple of months. Steady effort works better than a single big clean.

Will a water change get rid of hair algae?

Not on its own, but water changes help with nutrient control alongside removal and grazers.

Can I add a tang to eat it?

Only if your tank is big enough for that fish as an adult. Choose grazers that suit the tank first.