Filamentous (Hair) Algae in Aquariums: Causes and How to Get Rid of It
Long green strands clinging to plants, decor or the glass, growing back a few days after every cleanup, almost always signal a nutrient imbalance rather than simple neglect. A landmark review by Val H. Smith, published in 2003 in the journal Environmental Science and Pollution Research, showed that nutrient enrichment, nitrogen and phosphorus above all, drives a predictable rise in algal biomass across aquatic systems, including closed ones (study on Google Scholar). Find our other pointers in the care and grooming section.
Why does filamentous algae take hold?
Excess light paired with a surplus of nitrates or phosphates, often from overfeeding or infrequent water changes, creates ideal conditions for these fast-growing algae. A newly maturing tank, or a planted aquarium under-dosed on CO2 relative to its lighting, is especially prone to it.
Is pulling out the strands enough?
No, winding them around a toothbrush and removing them offers temporary relief, but without fixing the underlying nutrient imbalance, the strands return within days. Our guide to lighting settings helps restore a balance between light and available nutrients.
How do you fix the imbalance for good?
Cutting back the photoperiod, increasing the frequency of partial water changes and avoiding overfeeding all limit the nutrient supply that feeds the algae. A population of fast-growing plants competes directly with algae for the same resources, often a more effective lever than an anti-algae product alone.
Can fish or shrimp help?
Certain shrimp and algae-grazing fish readily eat young, tender strands, which slows their spread without replacing the need to fix the root causes. See our comparison of algae-eating fish and snails to pick a species suited to your tank.
- Excess light and nutrients: the combination that triggers the outbreak.
- Manual removal: temporary relief, not a lasting fix.
- More frequent water changes to limit nitrates and phosphates.
- Fast-growing plants compete directly with algae.
Frequently asked questions
Is filamentous algae dangerous for fish?
No, it poses no direct danger, though a dense tangle can trap small fish or fry.
Do you need to turn off the lights entirely to get rid of it?
No, cutting a few hours off the daily photoperiod is usually enough without harming the plants.
Does a chemical algaecide solve the problem for good?
Rarely on its own: without fixing the underlying light and nutrient imbalance, algae return once the treatment wears off.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.