Thread and Filamentous Algae: The Hair Algae Complex

Green hair and thread algae are a complex of look-alikes: Oedogonium, Rhizoclonium and Spirogyra. Learn to tell them apart, why they share causes, and how to clear them.

Green filamentous algae, the family of pests usually called hair or thread algae, all look broadly similar: like wet hair when lifted from the water. They are commonly grouped together because they share the same triggers and respond to the same controls. Several different genera hide under these names, and knowing which one you have helps you set expectations, especially for telling them apart from the much tougher Cladophora.

The look-alikes

• Oedogonium: a genus of filamentous, free-living green algae with unbranched filaments; young filaments attach to plants and surfaces by a holdfast cell, and it is often seen as short fuzzy growth on leaf edges.
• Rhizoclonium: green algae in the family Cladophoraceae with straight or curved, usually unbranched filaments bearing short rhizoids; it is cosmopolitan in fresh, brackish and marine water and tends to show up in immature tanks.
• Spirogyra: a filamentous charophyte green alga named for its spiral chloroplasts, commonly called water silk or blanket weed; it is a freshwater alga that often feels slippery and forms drifting masses.

Shared causes

Despite the different genera, hair and thread algae share the same underlying triggers. They tend to attach to damaged or stressed plants, so they are most common in new, immature tanks where plants are still adapting, and where light is high relative to the CO2 and nutrients available. Ammonia from an immature filter or disturbed substrate, excess organics and unstable CO2 all feed the problem. A healthy, well-maintained planted tank should be essentially free of them; their presence is a signal that the plants are struggling.

Control

• Fix the imbalance first: stabilise CO2, match light to your fertiliser and CO2, and improve flow so it does not simply return after removal.
• Manual removal: twirl the strands out with a toothbrush, then do a large water change to export floating fragments.
• Spot treatment: a liquid-carbon product or 3% hydrogen peroxide dabbed on affected areas can weaken stubborn patches.
• Grazers: Siamese algae eaters, Amano shrimp, mollies and Florida flagfish all browse filamentous algae and make good clean-up crew.