Hair algae in aquariums: causes, removal and how to stop it coming back

Hair algae is a sign that your tank has more light or nutrients than your plants can use. It is not dangerous to fish, so you have time: pull out what you can by hand, shorten the lights, and fix the imbalance behind it, or it will grow back within days.

This guide covers the different kinds of hair algae, why it grows, how to remove it, which algae eaters actually help, and how to rebalance the tank. For algae in general, see our aquarium algae guide.

How to identify it

Hair algae is a loose name for several kinds of filamentous algae: algae that grows as long, thin threads. When you lift it out of the water it hangs limp, like wet hair. The common types are:

Hair or thread algae: fine green strands, a few centimetres (1 to 2 inches) long or more, often growing from plant leaves, wood and rocks.
String or spirogyra-type algae: very fine, bright green and slimy. It feels slippery between your fingers and spreads fast.
Cladophora: stiffer, branching green threads that feel wiry. It often arrives tangled in new plants.
Fuzz algae: short green fuzz, under 1 cm (0.4 inches), on leaf edges and hardscape. It is the same problem at an earlier stage.

Not the same thing: black beard algae is dark grey to black and grows in short, stiff tufts. Staghorn algae is grey-green and branches like antlers. Cyanobacteria forms a slimy blue-green sheet that smells bad and peels off, not threads.

When it's a problem

Normal: a few strands here and there in a planted tank. Almost every tank has some algae.
Early: fuzz on leaf edges, or new strands appearing every week. Now is the easiest time to fix it.
Serious: mats that wrap around plants, block their light and trap food and waste. Plants start to suffer, and small fry or shrimplets can get tangled.
Emergency: hair algae on its own almost never is. But if the algae boom came after a sudden change (a dead fish, heavy overfeeding, a filter failure), test for ammonia and nitrite, because that is the real risk.

What to do right now

Remove as much as you can by hand. Twirl it around an old toothbrush, a bamboo skewer or your fingers, the way you'd wind spaghetti on a fork. Pull heavily covered leaves off the plant entirely.
Siphon out loose bits during a water change so they don't settle and regrow.
Do a 30 to 50% water change with dechlorinated, temperature-matched water to lower excess nutrients.
Cut the lighting period to about 6 to 7 hours a day, on a timer.
Stop dosing fertiliser for a few days while you test, especially any product high in iron.
Test your water: ammonia, nitrite, nitrate and, if you can, phosphate. Write the numbers down.

What to do next

Find the imbalance

Algae grows when plants can't use all the light and nutrients on offer. Hobby references point to three common causes: too much light, too much of one nutrient (iron is the classic one), and too few nutrients to match a long lighting period. That last one surprises people. In a tank with strong light and hungry plants, under-fertilising can cause hair algae, because the plants stall and the algae takes over.

Lots of light, few plants: reduce light intensity or hours, and add fast-growing plants (stem plants, floating plants) to compete for nutrients.
Heavy fertiliser dosing, especially iron: cut back. Some planted-tank references suggest keeping iron around 0.05 to 0.1 mg/L, but most beginners don't need to test iron. Halving the dose is a fair first step.
High nitrate or phosphate from fish waste: feed less (see overfeeding), vacuum the gravel and change water more often. See high nitrates.
Strong light with CO2 injection: make sure CO2 is steady through the whole lighting period and that flow reaches every corner. Unstable CO2 is a common trigger in high-tech tanks.

Experts disagree on phosphate. Some say high phosphate feeds algae; many planted-tank keepers say phosphate is rarely the cause on its own as long as plants are growing well. Test rather than guess, and change one thing at a time so you can see what worked.

Add algae eaters

Amano shrimp (Caridina multidentata) are the most widely recommended hair algae grazers. They pick at fine threads all day. They don't breed in fresh water, so they won't overrun the tank. Large fish will eat them, so choose tank mates with care. Don't feed the tank so heavily that the shrimp have no reason to graze.
Siamese algae eaters (Crossocheilus oblongus) eat hair algae and black beard algae when young. But they grow to about 15 cm (6 inches) within two years, need at least 100 litres (about 26 US gallons), jump, and often lose interest in algae as adults. Many shops sell look-alikes such as the flying fox, which is more territorial and less useful. The true species has a black stripe that runs right into the tail fin.
Mollies and Florida flagfish also graze hair algae, but check they suit your tank's temperature and water hardness.
Nerite snails help with film and spot algae on glass but don't eat long threads well.

Spot treatments (use with care)

Liquid carbon (glutaraldehyde-based products): turn off the filter for 10 to 15 minutes and squirt a small amount directly onto algae with a syringe. Affected algae often turns white or pink in a few days. Stay within the label's total dose for your tank. Cautions: it can melt Vallisneria, some mosses and some delicate plants, and overdosing can harm shrimp and snails.
Hydrogen peroxide (3%): used by some keepers as a spot treatment or as a short dip for plants and decor outside the tank. Rinse well before returning them. Overdosing in the tank can burn fish gills and kill shrimp, so beginners are better off dipping decor out of the tank.
Remove carbon from the filter only if you're using a real medication; algae products don't usually require it, but check the label.

Spot treatments only kill what's there. Without fixing light and nutrients, the algae comes back.

What to expect

New growth should slow within one to two weeks of rebalancing. Old algae doesn't disappear on its own, so keep removing it by hand. Give it four to six weeks before judging the results.

Products that help

Lighting timer: keeps the photoperiod short and consistent, the single cheapest fix.
Liquid test kit (nitrate, phosphate): shows whether nutrients are building up.
Glutaraldehyde-based liquid carbon: for syringe spot treatment of stubborn patches.
Algae scraper or old toothbrush: for winding out threads.
Gravel vacuum: removes waste that feeds algae.
Fast-growing or floating plants: compete with algae for light and nutrients.
Balanced, lower-iron fertiliser: for planted tanks that need feeding without overdoing iron.

How to prevent it

Keep lights on 6 to 8 hours a day with a timer. More light needs more plants and more nutrients.
Feed fish only what they finish in a minute or two.
Change 25 to 50% of the water weekly.
Plant heavily, including some fast growers.
Dose fertiliser to match plant growth, not the bottle's maximum.
Rinse or dip new plants, and quarantine them in a bucket for a week, to avoid bringing in Cladophora.
Keep a small clean-up crew, such as a few Amano shrimp.

Why it happened

Algae and plants compete for the same light and nutrients. When the balance tips (a new brighter light, more fish, more food, too much fertiliser, unstable CO2 or a big plant trim), algae responds faster than plants do. New tanks also get algae waves while they settle in.

When to get help

Ask an experienced planted-tank keeper or local fish club if the algae keeps coming back after a month of steady changes, or if you run CO2 and can't get stable levels. If fish are gasping or acting sick at the same time, the algae is not the main problem: test the water and see fish gasping at the surface.

In the game

Multiple Tank Syndrome doesn't have a hair algae card yet.