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Troubleshooting

Cloudy water: causes and fixes

Cloudy water has several causes with different fixes. Match the cause before reaching for a clarifier.

2 min readUpdated September 27, 2026By FishicianPublished August 27, 2026How we source

The short answer

  • Look at the colour. Colour and timing suggest causes, but cannot identify them on their own.
  • White in a new tank may be a bacterial bloom or suspended dust. Check water quality, aeration and fish before deciding to wait.
  • Haze after cleaning is usually fine particles. Fine mechanical filtration clears it.
  • Green water can be floating algae. Review light, feeding and water tests. Keep light and nutrients suitable for plants or photosynthetic corals.
  • Brown is usually tannins. Mostly cosmetic. Activated carbon clears it if you prefer.
A home aquarium with pale cloudy water obscuring the plants at the back
Cloudiness has several possible causes; the appearance alone cannot establish one.
Match the colour to the cause
White / grey
  • Bacterial bloom or fine dust
  • Common in new or disturbed tanks
  • Test ammonia and nitrite
Green
  • May be free-floating algae
  • Review light and nutrient inputs
  • Protect plant and coral needs
Brown / yellow
  • Tannins from wood or leaves
  • Usually harmless
  • Carbon if you want it clear

White haze: bacterial bloom

  • Appears within a day or two of setup or a big substrate disturbance.
  • A bloom can increase oxygen demand. Ammonia or nitrite may indicate an immature or disturbed biofilter; test rather than judging by appearance. See cycling.
  • Do not add extra food or stack products. If fish are distressed or tests show a problem, act promptly; suitable water changes may be needed.

Fine particles

  • Substrate dust or disturbed decor settles slowly and tends to persist.
  • A fine filter pad clears it faster than waiting.
  • Rinse new substrate well and go easy on the gravel vacuum after rescaping.

Green water

  • Floating algae, not the kind on the glass. Scraping won't touch it.
  • Review the lighting schedule, direct sunlight, feeding and stocking. Correct excesses while meeting the needs of your animals and plants.
  • Reef tanks: photosynthetic corals need appropriate light and nutrients too. Do not apply a blanket blackout or drive nitrate and phosphate to zero to clear green water. Identify the problem and adjust for the corals you keep.
  • UV or fine filtration removes it faster, but fix the inputs or it returns. See algae control.

What not to do

  • Stack clarifiers, or mix one with another treatment without reading both labels.
  • Do a huge emergency water change on a new or crowded tank for cloudiness alone.
  • Assume it's the same cause as last time.

Timing tells you a lot

  • Sudden haze within a day or two of setup: usually a bacterial bloom.
  • Haze after rescaping or vacuuming may be particles; persistent haze or distressed fish needs investigation.
  • Green building up over days: light and nutrients.
  • Brown after adding wood or leaves: tannins.
  • Note what changed recently before choosing a fix.

Sources

Each source is listed with the part of this guide it supports.