Green hair algae grows when light and nutrients exceed what your plants actually use. Cut the photoperiod to 6-7 hours first, then adjust dosing if it persists. Combine manual removal (twirling strands onto a toothbrush) with grazers like Amano shrimp for a result that holds — removal alone, without fixing the light or nutrient imbalance, is temporary.
Green hair algae forms long, fine filaments that attach to plant leaves, hardscape, and equipment, and it's one of the most common complaints in newly planted tanks. It isn't a random infestation — it's a direct symptom of imbalance between light, nutrients, and plant consumption. Whenever light and nutrients are available in excess of what the plants are actively using, algae is what fills the gap. Treating the symptom without addressing that imbalance is why hair algae so often returns after being scrubbed away.
Of the two variables, excess light is the more frequent root cause — either too many hours of photoperiod, or intensity too high for the plant mass in the tank. The standard first move is cutting the photoperiod to 6-7 hours and holding it there for two weeks before making any other change. This isolates whether light is the driver before touching the fertilizing routine, which is riskier to adjust aggressively since plants depend on it too.
If cutting light doesn't resolve it, nutrient dosing — particularly phosphate and nitrate — is the next variable to examine. This often traces back to a fertilizer routine added without adjusting the light schedule to match, or a tank that's under-planted relative to how much light and nutrient it receives. More plant mass competing for the same nutrients starves algae of what it needs to spread.
An overstocked tank can push the same imbalance from the fish side rather than the fertilizer bottle — more fish means more waste, which means more ammonia converted into nitrate through the nitrogen cycle, feeding the same surplus even without any deliberate dosing. If dosing looks reasonable but algae persists, checking bioload relative to plant mass is worth doing before assuming the fertilizer routine itself is the problem.
Twirling a soft toothbrush or dedicated algae brush into the strands winds them up for removal, taking more biomass out than scraping does and reducing the spore load left in the tank. Pair this with a water change on the same day — disturbing algae releases spores into the water column, and diluting that water reduces regrowth from the removal process itself.
Amano shrimp are the most reliable and widely available grazer for hair algae, and true Siamese algae eaters (not the commonly mislabeled Chinese algae eater sold under that name) eat it well while young. Neither fixes the underlying light or nutrient imbalance on its own — they reduce the visible algae while you correct the cause, not instead of correcting it.
Stocking grazers before the light or nutrient issue is fixed also has a downside worth knowing: once hair algae is scarce, both Amano shrimp and SAEs largely stop eating it and turn to other food sources, including tender new plant growth in some cases. Plan grazer stocking as a supporting measure alongside the fix, not a standalone algae-control strategy you can rely on indefinitely.
Green hair algae forms long, distinct filaments you can grab between two fingers and pull as a strand — this distinguishes it from green spot algae (hard, disc-shaped, adheres tightly to glass) and from blue-green algae, which isn't a true algae at all but a cyanobacteria that forms a slimy sheet rather than filaments and typically signals a different problem (poor flow and organic buildup rather than light/nutrient excess). Misidentifying the type leads to treating the wrong cause — a cyanobacteria bloom, for instance, generally needs a blackout and improved flow, not a photoperiod cut.
Once the photoperiod or dosing adjustment is in place, expect existing hair algae to stop spreading within about a week, but full clearing of established growth typically takes two to four weeks of consistent manual removal alongside the fix. Patience here matters — reversing the light or nutrient change back too soon, out of frustration that algae is still visible, resets the timeline and often makes the regrowth worse than the original bloom. Track progress with photos taken from the same angle each week rather than relying on memory — hair algae recedes gradually enough that day-to-day comparison can make a real improvement feel like no progress at all.
Our complete freshwater guide covers lighting, dosing, and plant selection together — the framework that prevents most algae issues before they start.