Hair and thread algae: what causes it, and what clears it

“Hair algae” is not one organism. It is a catch-all for several unrelated filamentous algae that share a look: fine green strands that drape over plants, hardscape and equipment. Getting rid of them is much easier once you know which one you have and, more importantly, what tipped the balance to let it take hold.

The common types under one name

Most “hair algae” is one of a few filamentous green algae (division Chlorophyta):

• Soft green hair algae (often Oedogonium and relatives) — short, soft, fuzzy growth on leaves and glass. The most common, and the easiest to remove and to graze down.
• Thread / silk algae (often Spirogyra) — long, slippery, bright-green threads that feel silky and slide between your fingers. Grows fast in high light.
• Cladophora — the odd one out: coarse, branched, and tough, with a faint musty smell. It is the same genus as the marimo “moss ball,” and its branching structure makes it far harder to remove and to graze than the soft kinds.

One lookalike is worth separating out: staghorn algae is grey-green and branches like tiny antlers, and despite the colour it is a red alga — closely related to black beard algae, not to the green hair algae — and it responds to the same measures. If your “hair algae” is stiff, grey-green and forks like antlers, treat it as black beard algae instead.

A balance, not a single cause

Filamentous algae are always present in the water as spores; whether they take over is a question of whether your plants are outcompeting them. Healthy, actively growing plants suppress algae through fast nutrient uptake, shading, and (probably) chemical means. Anything that weakens plant growth relative to the light on offer gives algae the opening. In practice, the recurring triggers are:

The honest summary: hair algae is a signal that light is outrunning your plants’ ability to grow, usually because CO₂, nutrients or plant mass are lagging — or that an ammonia source has handed the algae a head start. Fixing the balance is the real cure; everything else buys time.

The order of operations

Do these together, roughly in this order:

1. Remove what you can by hand. Soft hair and thread algae wrap neatly around a toothbrush or twig if you twirl it; Cladophora has to be picked out. Removing the bulk immediately reduces the spore load and lets treatments and grazers work on what is left.
2. Cut the light back to match the plants. Shorten the photoperiod (six to seven hours is plenty for most tanks) and, if your light is dimmable, lower the intensity. This is the highest-leverage change — see how much light aquarium plants actually need.
3. Fix the underlying weakness. Stabilise CO₂ if you inject it, make sure the plants are actually fed (consistent, not feast-and-famine, dosing — see nutrient uptake and dosing), and add fast-growing stems to soak up the surplus.
4. Deal with any ammonia source. If the tank is new, it is still cycling — see the nitrogen cycle in a closed aquarium. Otherwise, check for overfeeding, overstocking, or dead patches in the substrate.
5. Spot-treat the stubborn bits. With the filter off, apply 3% hydrogen peroxide or a glutaraldehyde product (Seachem Excel, Easy-Life Easycarbo) directly onto the algae by syringe, leave five to ten minutes, then restart the filter. Treated filaments pale and break down over a few days.

Let the clean-up crew help

Grazers keep hair algae in check far better than they clear a heavy outbreak, so add them after you have knocked the bulk back. Amano shrimp are the most effective all-rounder for soft hair and thread algae; Siamese algae eaters (true Crossocheilus) will graze filamentous algae and are among the few fish that touch staghorn and BBA. Nerite snails deal with films and diatoms more than long hair. Note the honest limit: Cladophora is largely unpalatable to the usual clean-up crew — it is a manual-removal and spot-treatment job.

Keeping it gone

Once the tank is clear, hair algae stays away when the balance holds:

• Match light to plant demand. More light is not better; it is simply more demand the plants must meet before the surplus feeds algae. Most tanks do well on moderate light and a six-to-eight-hour photoperiod.
• Feed the plants consistently. Steady dosing beats sporadic heavy dosing; a plant that never runs short grows steadily and stays competitive.
• Keep plant mass high. A densely planted tank with fast growers is the best algae control there is.
• Avoid ammonia. Do not overfeed, do not overstock, and clean filters in old tank water rather than stripping the biofilter.
• Keep flow and maintenance regular. Good circulation and routine trimming and water changes stop debris and dead spots from feeding an outbreak.