How to get rid of hair algae in your aquarium

What is hair algae?

Hair algae — also called thread algae, filamentous algae, or string algae — is a collective name for several species of green filamentous algae (most commonly Spirogyra, Pithophora, and Cladophora) that form long, hair-like strands. It grows on plants, substrate, driftwood, rocks, and filter intake pipes. In a bad outbreak, it forms thick mats that smother plants, trap debris, and make the whole tank look like a green fur coat.

Unlike green spot algae (which is hard and disc-shaped) or blue-green algae (which is slimy and smells), hair algae is soft, stringy, and pulls away in clumps when you grab it. That’s both its weakness and part of why it spreads — fragments float off and seed new growth elsewhere.

What causes hair algae? (Data first)

Before treating an outbreak, identify the driver. Hair algae almost never appears in a well-balanced tank. It appears when something in the system is out of balance. The most common causes:

• Excess nutrients with insufficient plant mass. Elevated nitrate and phosphate, combined with a lightly planted or unplanted tank, gives algae everything it needs with no competition. Check your nitrate levels — anything above 20 ppm sustained in a planted tank is a red flag.
• Too much light for the plant load. If your lights run 10–12 hours a day in a sparsely planted tank, algae gets the energy it needs while your plants can’t absorb nutrients fast enough to outcompete it. Reduce to 6–8 hours.
• CO₂ fluctuations. In CO₂-injected tanks, an unstable CO₂ supply (dropping overnight, fluctuating during the day) stresses plants and creates an algae-friendly window.
• New tank instability. New tanks lack mature plant root systems and established bacterial colonies — algae exploits that gap.
• Overfeeding. Uneaten food decays and releases phosphate. If you’re feeding more than fish can eat in 2–3 minutes, you’re fertilising algae.
• Infrequent water changes. Accumulated nutrients fuel algae. Regular water changes — typically 20–30% weekly — keep nutrient levels in check. See our water change frequency guide.

Step 1: manual removal

Before anything else, remove as much hair algae as you can manually. Use a clean toothbrush, an algae scraper, or simply your fingers. Twirl the toothbrush into mats of algae — it wraps around the bristles like spaghetti on a fork. Work systematically: plants first, then hardscape, then substrate. Remove everything you can see. Get as close to zero mass as possible before your cleanup crew takes over.

Do a 30–40% water change after manual removal to suck out floating fragments and reduce nutrient load. Clean the glass and wipe intake pipes.

Step 2: fix the root cause

This is the step most people skip — and why their algae comes back. Manual removal and biological control are maintenance, not cures. The cure is fixing the imbalance that allowed the algae to grow:

• Reduce photoperiod to 6–7 hours. Set a timer.
• Reduce feeding. One feeding per day, remove uneaten food.
• Increase plant density. Fast-growing stem plants (hornwort, water sprite, hygrophila) are particularly effective at outcompeting algae for nutrients.
• Do 2–3 consecutive daily water changes of 30%+ to drop nutrient levels rapidly.
• If you have CO₂ injection, stabilise it — use a controller if possible.

Step 3: the cleanup crew

Biological control is where most people either succeed or fail. Add the right animals in the right numbers, and hair algae becomes a self-limiting problem. Add the wrong species in the wrong quantities, and nothing changes.

Amano shrimp — the gold standard

Caridina multidentata, the Amano shrimp, is the single most effective biological control for hair algae in the freshwater hobby. Named after Takashi Amano — the legendary aquascaper who popularised their use in planted tanks — these shrimp are methodical, relentless, and surprisingly large for a shrimp (up to 5 cm). They will spend every waking moment picking through algae, consuming hair algae, thread algae, and soft green algae at a rate that genuinely makes a visible difference within days of introduction.

What makes Amano shrimp special compared to other algae-eaters:

• Size. At 4–5 cm, they are big enough to tackle thick mats of hair algae that dwarf shrimp (like cherry shrimp) simply can’t handle.
• Diet specificity. Amano shrimp prefer algae over anything else when it’s available. Many fish that are sold as algae eaters will ignore algae the moment flake food is an option. Amano shrimp won’t.
• Non-destructive. Unlike some algae-eating fish, Amano shrimp don’t eat healthy plant tissue. They graze biofilm and algae from plant leaves without damaging the plant itself.
• Community-safe. They are peaceful and do not bother fish. They will be eaten by larger fish (cichlids, large tetras, bettas), so check compatibility before adding.

How many do you need? The rule of thumb in planted tank circles is 1 Amano shrimp per 5 litres (roughly 1 per gallon) for an active outbreak. So a 60-litre tank needs 10–12 shrimp during an outbreak — not 3. Under-dosing is the most common mistake. For maintenance after the outbreak is cleared, 1 per 10 litres is sufficient.

One important note: Amano shrimp cannot breed in freshwater. They require brackish conditions to reproduce. This means your population will not explode, which is actually a feature — you can control numbers easily.

Nerite snails

Nerite snails are superb at eating short algae from hard surfaces — glass, rocks, driftwood, broad plant leaves. They are not effective against long, established hair algae mats. Use them as a complement to Amano shrimp: shrimp for the hair algae, nerites for green dust and spot algae on surfaces. 1–2 nerites per 10 litres. See our algae guide for how each type of algae responds to different grazers.

Otocinclus catfish

Otocinclus are outstanding biofilm and green algae grazers on plant leaves, but they have a small, soft mouth and are not effective against thick hair algae. They complement Amano shrimp rather than replacing them. They are delicate fish and need a fully cycled, well-established tank.

Siamese algae eaters (Crossocheilus oblongus)

True Siamese algae eaters (not Chinese algae eaters, which are a different and often aggressive fish) will eat some hair algae, particularly when young. They are one of the few fish that will tackle black beard algae as well. However, they grow to 15 cm and become less enthusiastic grazers as adults. In a large tank (100 litres+), 2–3 true SAEs can contribute meaningfully. Confirm species before buying — the look-alike species don’t eat algae.

Mollies

Mollies are underrated hair algae grazers. They will actively pick at filamentous algae, particularly in tanks with harder, slightly alkaline water where mollies thrive anyway. Not suitable for softwater setups or tanks with shrimp (mollies may harrass shrimp). But in the right context, a small group of mollies will make a noticeable dent in mild outbreaks.

What about flying foxes, plecos, and mystery snails?

Flying foxes (often sold as SAEs) eat algae as juveniles but become largely territorial and unhelpful as adults. Common plecos grow enormous and produce waste that worsens nutrient problems — avoid for hair algae control. Mystery snails graze soft algae but are not effective against hair algae.

Chemical and hydrogen peroxide treatment

For severe, entrenched outbreaks, spot treatment with diluted hydrogen peroxide (H₂O₂) is a legitimate and widely-used approach. Standard method: using a syringe, inject 1–2 ml of 3% H₂O₂ directly onto the affected area while the filter is off. Wait 5 minutes, then resume flow. H₂O₂ decomposes rapidly into water and oxygen — there is no persistent residue. Repeat every 2–3 days for up to two weeks.

Be conservative. H₂O₂ at high concentrations or in open water will harm shrimp and delicate plants. Spot treatment to the algae directly, not a broad tank dose. This is a tool to knock back a severe outbreak, not a replacement for fixing root causes.

Liquid carbon products (like Seachem Excel or similar) are also used — they act as a mild algicide at dosing levels. Add to the affected area with a syringe for spot treatment. Can also harm shrimp in high concentrations — use carefully.

The long-term solution

Hair algae is a tank balance problem. Fix the nutrients, fix the light, add enough Amano shrimp, and the algae runs out of the conditions it needs to thrive. A well-planted, well-maintained tank with an active cleanup crew — particularly Amano shrimp — is one that hair algae struggles to establish in at all. The biology does the work if you set the system up correctly.

Track your water quality consistently to catch imbalances early. If you log parameters in App-aquatic, you’ll spot a creeping nitrate climb — the most common precursor to hair algae — long before the algae appears.