Green water is one of the most common aquarium algae problems. What it is: a bloom of free-floating single-celled algae (mostly Chlorella + Chlamydomonas). The water turns from clear to pea-soup green over 3-7 days. Causes: 1) Direct sunlight on tank - the #1 trigger. Move tank or close curtains. 2) Excess nutrients (high nitrate + phosphate) - cut feeding 50%. 3) Long photoperiod (10+ hours) - reduce to 6-8 hours. 4) Spike from disturbed substrate - algae spores released. 5) New tank "diatom phase" sometimes turns green if silicates run out. Solutions: 1) **3-day blackout** - cover tank entirely (no light at all), turn off light, fish okay for 3 days, cover for 4 days = algae dies + settles. Restart light at 6 hours. 2) **UV sterilizer** - 9-13W kills algae passing through. Clears tank overnight. Most reliable. 3) **Fast-growing stem plants** - hornwort, water sprite, frogbit consume nitrate + phosphate before algae can. 4) **Daphnia introduction** - filter feeders eat algae cells. 5) **Reduce feeding + larger water changes** - less nutrient input. Avoid: chemical "algae killers" (copper-based, kills inverts) + flocculants (just precipitates algae temporarily).
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