Hair, thread and fuzz algae: telling them apart and what helps

Four kinds of green filaments

Growers group green filamentous algae by how they grow, because the remedies differ:

Kind	What it looks like	Grower notes
Thread algae	Long, soft, loose green threads that form bushy cushions and drift in the current	Not anchored by holdfasts; eaten readily by Amano shrimp [1]
Hair algae	Dense, carpet-like coats of short filaments over leaves and decoration, usually the genus Oedogonium	Soft forms are grazed; one short, bristly form is rare but stubborn [2]
Fuzz algae	Short single filaments on plants, decoration and glass	Typical in weeks 4–8 of a new tank; a slight permanent population is normal [3]
Cladophora	Short, rigid, branched filaments in dense tufts, often in carpeting plants and mosses; smells of mushrooms	Poorly grazed; barely responds to fertilizer changes [4]

The cycling phase

Most of these algae appear while a tank is still cycling, when the microbial community and the balance between algae and plants are not yet settled; as plant mass grows, the plants gain the upper hand ^{[1, 2]}. Aquasabi’s advice for that period is to plant densely from the start, preferably with fast-growing stem plants, and to supply the full range of nutrients and CO2 ^[1]. For new tanks it also suggests using only a third of the iron fertilizer dose recommended on the label ^[1].

Triggers in an established tank

When thread or hair algae break out in a tank that was stable, the sources point to four things:

• Light out of step with plants and nutrients. A new, stronger light, new tubes, extra daylight from a move or the season, or longer hours can all trigger an outbreak. Aquasabi’s rule is that more light always needs more nutrients ^{[1, 2]}.
• Nitrate shortage. Plants stall when nitrogen runs short, and green algae take over the unused nutrients. Aquasabi recommends testing nitrate and raising it alone, without extra potassium or phosphate, if it is low ^{[2, 1]}. The fertilizer dosing calculator gives the grams of potassium nitrate for a target.
• Too little CO2. Aquasabi’s target is about 20–30 mg/L, read on a drop checker ^[1]; our drop checker guide shows what its colors mean in numbers.
• Warm water. Tropica says temperatures above about 26 °C almost always cause algae problems, that thread algae especially like heat, and that lowering the temperature to 22 °C or less is worth trying ^[6].

Photoperiod: the ramp-up schedule

Both growers limit the hours of light. Tropica’s general maximum is 10 hours a day ^[6]. Aquasabi starts new tanks at 6 hours and adds 30 minutes a week, ending at 8–10 hours for medium to strong lights or 10–12 hours for weaker ones ^[1]. At that rate, getting from 6 to 10 hours takes eight weeks. The photoperiod guide compares these limits with the light ratings in grower data.

Removing what is there

• Water changes. Tropica recommends changing water more often than usual; Aquasabi recommends at least 50 % a week as routine ^{[5, 1]}.
• Grazers. Tropica’s remedy for filamentous algae is a large population of Amano shrimp, up to 2 per 5 L of water for a transitional period ^[5]. Aquasabi adds dwarf shrimp and, for hair algae, ramshorn snails ^{[1, 2]}.
• By hand. Cut off badly infested leaves ^[2]; for Cladophora, pick tufts out with long tweezers and switch the current off first so fragments do not spread ^[4].
• Chemicals. Tropica generally advises against removing thread algae with chemical preparations, because some are harmful to aquarium plants ^[5]. Aquasabi describes spot treatments with liquid carbon or hydrogen peroxide ^[2]; this site does not give doses for those, so follow the product label.

Cladophora is the exception to all of the above: Aquasabi reports that it usually arrives as fragments on plants from other aquariums, and that the only certain way to remove it is to restart the tank without reusing plants or substrate ^[4]. For that reason Aquasabi recommends buying plants from emersed cultivation or tissue culture rather than low-priced imported bundles ^[4]; the emersed-to-submersed guide explains those forms.