Cloudy aquarium water has several distinct causes - identify the color first. White or grey haze: bacterial bloom. Common in newly-set-up tanks (7-14 days post-setup) when heterotrophic bacteria temporarily overpopulate before nitrifiers take over. Don't panic - clears on its own in 3-7 days. Don't do massive water changes (slows cycling). Green water: single-celled algae bloom. Caused by too much light (especially direct sunlight), excess nitrate + phosphate, or a recent dechlorinator overdose that fertilized algae. Solution: 3-day blackout (cover tank, lights off), reduce photoperiod to 6-8 hours, do a 30% water change. UV sterilizer (9-13W) clears it overnight. Yellow tint: dissolved organic compounds (DOCs) from fish waste, decaying food, or driftwood tannins. Run activated carbon or Purigen for 2-3 weeks. Brown haze: diatom bloom (silica-fed). Common in tanks under 4 months. Clears as silicates deplete. Otocinclus and Nerite snails consume diatoms.
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