Algae guide
Hair Algae in Aquarium: Why It Grows and How to Remove It
Last updated: May 2026
- What it is: Filamentous green macroalgae that mats over mosses, stem plants, and hardscape - looks like wet green cotton tangled around everything you care about.
- Root cause: A mismatch between light intensity and carbon availability. Too much light, not enough CO₂ (or both), and hair algae fills the gap your plants left open.
- Severity: Moderate (2-3 out of 4). Cosmetically devastating and capable of smothering moss beds within weeks, but it won't kill fish directly.
- Primary fix: Manual removal first, then correct the light/CO₂/nutrient triangle. Fauna like Amano shrimp accelerate cleanup but won't fix the underlying cause alone.
- What most guides miss: Slashing nitrates to zero is a dead end - hair algae thrives in low-nutrient tanks. Balanced macros and disciplined photoperiod management matter far more.
- Timeline: With correct adjustments, active growth typically slows within two to three weeks. Full clearance in planted tanks takes four to six weeks.
What hair algae actually is
Hair algae isn't one species - it's a loose term for several filamentous green algae genera, most commonly Spirogyra, Pithophora, and Oedogonium. All three produce the same characteristic thin, thread-like strands that clump into matted tufts. Under your fingers it feels silky when wet and dries into a brittle crust. On mosses like Taxiphyllum barbieri (Java moss) or fine-leaved stems like Rotala rotundifolia, it weaves itself into the plant so tightly that pulling it out damages leaves.
Unlike spot algae or green dust algae - which colonise hard surfaces - hair algae specifically targets plant surfaces and substrate. That distinction matters because it tells you exactly what's wrong: your plants aren't winning the competition for available light and carbon. The algae is exploiting the surplus your plants can't use.
Why it's growing in your tank
Every hair algae outbreak traces back to the same triangle: light, CO₂, and nutrients. Tip the balance wrong and you invite it in. Here's how each leg of that triangle contributes.
Excess or mistimed lighting
This is the most common driver. Most planted tanks run 8-10 hours of light daily, but the intensity matters as much as duration. A high-output LED set at 80-100% for eight hours in a low-tech tank is far too much. Hair algae is extremely efficient at harvesting light - it outcompetes rooted plants during peak-intensity midday hours when CO₂ is already being depleted. The fix isn't always a shorter photoperiod; sometimes it's a dimmer one.
Low or fluctuating CO₂
In a pressurised CO₂ setup, unstable injection is a major trigger. If your CO₂ drops off in the evenings or runs inconsistently, plants slow their uptake, excess light goes unused, and filamentous algae capitalises immediately. In low-tech tanks with no CO₂ injection, the answer isn't to add CO₂ - it's to reduce light intensity to match what plants can actually process with ambient carbon levels.
The nitrate starvation myth
Plenty of advice online tells you to starve your tank of nutrients to beat algae. It sounds logical. It's wrong. Hair algae is metabolically lean - it grows in nutrient-poor conditions that would stall most aquatic plants. Cutting nitrates to near-zero doesn't kill hair algae; it weakens your plants and shifts the competitive balance further in algae's favour. Target nitrates at 10-25 mg/L and phosphates at 1-2 mg/L. Healthy, fast-growing plants are your best long-term weapon.
Doing large emergency water changes to crash nutrient levels. This stresses fish, disrupts your nitrogen cycle, and does nothing to address light or CO₂ - the actual cause. If you want to understand how water changes interact with tank chemistry, read our guide on how often to do water changes.
New tank syndrome and unstable cycling
Hair algae frequently explodes in tanks that are newly set up or recently re-scaped. An uncycled or partially cycled tank has no established plant root mass, no stable bacterial colony, and fluctuating parameters - perfect conditions. If you're dealing with hair algae in a tank under three months old, fixing your cycle may be more useful than any algae-specific intervention. See our guide on how to cycle a fish tank if you're not sure where you stand.
Manual removal: the non-negotiable first step
No chemical or biological treatment works well if you leave the bulk of the algae in place. Start by removing as much as physically possible before you do anything else.
For free-floating tufts and substrate mats, a wooden chopstick or toothbrush works brilliantly - twirl it like spaghetti and the strands wind onto it. For algae embedded in moss, gentle targeted extraction with tweezers is slower but preserves the plant. For heavily infested moss clumps, trimming the whole section back hard and letting it regrow is often faster than trying to pick it clean strand by strand.
After a manual harvest session, do a 30-40% water change and clean your filter intake - loosened filaments clog mechanical filtration fast.
Run a fine mesh net just below the water surface immediately after disturbing algae mats. Loose strands float before they settle, and catching them now saves you from dealing with new colonies seeding across the tank.
Glutaraldehyde spot treatment
Glutaraldehyde (sold under names like Seachem Excel or as liquid carbon supplements) is an effective spot treatment for persistent hair algae when used correctly. It works as a carbon source for plants at low doses and as a direct algaecide at higher concentrations applied directly to algae.
The workflow: turn off circulation. Draw up a small amount of undiluted or lightly diluted glutaraldehyde into a syringe (1-2 ml per treatment area). Position the tip 1-2 cm from the algae mat and dose slowly directly onto it. Leave circulation off for 10-15 minutes, then restart. You'll see affected hair algae turn white or pale pink within 24-48 hours - that's cell death. Remove the bleached material manually.
Glutaraldehyde is toxic to invertebrates at higher concentrations. If you have shrimp or snails, halve the dose and increase circulation restart time to 5 minutes maximum off. Mosses and fine-leaved plants like Utricularia graminifolia can also show temporary melt at high doses - use a diluted solution (1:4 with tank water) when treating near sensitive plants.
Biological control: what actually works
The right fauna can accelerate clearance significantly - but only after you've addressed the root cause. Algae eaters are maintenance crew, not problem solvers.
Amano shrimp (Caridina multidentata)
The most effective invertebrate for hair algae. Amanos are big enough (up to 5 cm) to physically shear and consume filamentous algae that smaller shrimp like Neocaridina barely touch. In a heavily infested 60-litre tank, 10-15 Amanos working overnight can clear surprisingly large areas. They won't eat plants unless they're starving, and they're peaceful with most community fish. Stock at roughly one Amano per 4-6 litres in an outbreak scenario, then reduce once algae is controlled.
Florida flagfish (Jordanella floridae)
One of the few fish that genuinely eats filamentous algae, including hair algae. They work best in slightly warm, harder water (22-26°C / 72-79°F, pH 7.0-8.0). The honest caveat: males can be aggressive toward smaller tankmates, and they occasionally nip fine-leaved plants when algae is scarce. Keep them well-fed and give them space - a pair in a 60-litre minimum. Not a beginner fish; check compatibility against your existing stock using a free stocking calculator before buying.
Siamese algae eaters (Crossocheilus oblongus)
Often recommended, but results on hair algae specifically are inconsistent. Young specimens graze it; adults (over 8-10 cm) frequently lose interest and switch to biofilm and leftover food. If you already have them, great. Don't buy them specifically for a hair algae outbreak.
Algae eaters are frequently oversold as solutions. In a tank where the light/CO₂ balance is genuinely corrected, a handful of Amano shrimp will clean residual algae efficiently. In a tank where the root cause is still active, you'd need impractical stocking numbers to keep pace with growth - and you'd still have the underlying problem.
Correcting your photoperiod and light intensity
The goal isn't to plunge the tank into darkness - that shocks plants and achieves little. The goal is to reduce light energy to a level your plants can fully utilise at their current CO₂ and nutrient levels.
If you're running a high-intensity LED at 80% for eight hours, try this: drop intensity to 50-55% and introduce a midday ramp-down (sometimes called a "siesta schedule" - full intensity for three hours, dim to 20% for two hours, back to full for three hours). This mimics natural midday light depletion and disrupts algae's peak-growth window without stressing plants.
If you're running a low-tech tank with no CO₂, keep total photoperiod at six to seven hours maximum and intensity moderate. Hair algae almost always signals that your light is outpacing your carbon availability. Dial back illumination by roughly 35% from current output and observe over two weeks - don't make multiple changes simultaneously or you won't know what worked.
Reducing peak PAR output by approximately 35% is typically enough to shift competitive advantage back to rooted plants without causing plant melt or stunted growth.
Water parameters and ongoing management
Stable parameters matter more than perfect parameters. Hair algae exploits fluctuation - swings in CO₂, temperature changes that alter plant metabolism, or erratic fertiliser dosing all create openings. Keep temperature steady in the range appropriate for your livestock (most planted community tanks: 23-26°C / 73-79°F), and dose fertilisers on a consistent schedule rather than reacting to visible deficiency.
For detailed guidance on what to measure and why, the aquarium water parameters guide covers targets for planted tanks specifically. And if you want to track test results over time rather than relying on memory, App-aquatic's parameter logging makes it straightforward to spot trends before they turn into problems.
Log your nitrate, phosphate, and pH readings weekly during an active hair algae outbreak. Pattern recognition - not a single snapshot - tells you whether your adjustments are working. Two weeks of stable or improving readings is a genuine signal; one good test result after a water change isn't.
What to do when nothing seems to work
If you've been manually removing, adjusting light, and running biological control for four weeks with no improvement, stop and audit these three things before doing anything else:
1. Check your light spectrum and age. LEDs don't burn out visibly - they shift spectrum. A three-year-old LED unit may be putting out more red-heavy light than it did when new, which disproportionately favours algae. If your unit is over two to three years old and heavily used, spectrum shift may be contributing.
2. Test for ammonia and nitrite. Elevated ammonia or nitrite alongside hair algae often signals a cycling issue rather than a nutrition/light problem. An unstable nitrogen cycle creates exactly the conditions hair algae thrives in. If you're unfamiliar with cycling, the nitrogen cycle guide is worth reading end to end.
3. Examine your substrate and hardscape. Decomposing plant matter trapped in substrate releases nutrients in pulses. Siphon the substrate properly during water changes, and check that any driftwood or rocks aren't leaching unusual compounds (some volcanic rocks can affect pH unpredictably).
Track your parameters and get care reminders with App-aquatic.
Get the free appWill a blackout kill hair algae?
A 3-day blackout (tank completely covered) weakens hair algae but rarely eliminates it, and it stresses plants simultaneously. It's a short-term intervention, not a fix. The algae often bounces back within a week once light returns if you haven't corrected the underlying cause. If you try a blackout, combine it with a large manual removal session immediately before and correct your photoperiod when you bring light back.
Can hair algae kill my fish or plants?
It won't directly harm fish. It can smother mosses and slow-growing carpeting plants by blocking light access, eventually killing sections if left unchecked. Fine-leaved stems can be strangled by heavy infestations. The indirect risk to fish is a degraded environment - heavily algae-choked tanks accumulate debris and reduce water flow around filters.
How many Amano shrimp do I actually need?
For an active outbreak in a 60-litre tank, aim for 10-15. In a 120-litre tank, 20-25 is realistic. Most stocking advice dramatically underestimates the numbers needed for meaningful algae control. They're peaceful and relatively cheap - buying enough to do the job properly is worth it. Once algae is cleared, the population can be reduced or left in place as ongoing maintenance.
Is hair algae the same as staghorn algae?
No. Staghorn algae (Compsopogon sp.) is a red algae that grows in single thick grey or greenish strands with a forked, antler-like shape. It's tougher, doesn't mat the same way, and requires a different treatment approach - it's highly responsive to glutaraldehyde but resistant to most biological grazers. Hair algae