Cloudy Aquarium Water: Causes & Fixes
The colour of your cloudy water tells you exactly what’s wrong. White, green, brown, and yellow cloudiness each have different causes — and different fixes. Here’s how to diagnose and clear your tank fast.
Diagnose By Colour
☁️ White or Grey Cloudiness
Cause: Bacterial bloom — most common in new tanks during the nitrogen cycle. Billions of bacteria colonising the water column cloud it white or grey. Completely normal and harmless.
Also possible: Substrate dust from unwashed gravel or sand.
- In a new tank: wait 1–2 weeks. The bacteria settle naturally once the tank cycles.
- Substrate dust: do a 30% water change, rinse substrate before adding next time.
- Don’t add chemical clarifiers to a cycling tank — they can remove beneficial bacteria.
- Increase surface agitation to improve oxygen levels during the bloom.
💚 Green Cloudiness
Cause: Green Water — a bloom of single-celled algae (phytoplankton) in the water column. Caused by too much light, excess nutrients (phosphate, nitrate), or direct sunlight hitting the tank.
- Reduce photoperiod to 6–7 hours temporarily.
- Move tank away from windows with direct sunlight.
- Do a 3-day blackout (cover tank completely) — kills most algae.
- Use a UV steriliser — most effective long-term solution for green water.
- Do 30% water changes every other day to export nutrients.
🟤 Brown or Yellow Cloudiness
Cause: Tannins leaching from new driftwood, leaves, or Indian almond leaves. Completely harmless — many fish (bettas, discus) actually prefer tannin-stained water as it mimics their natural habitat.
- Soak new driftwood in a bucket for 1–2 weeks, changing water daily, before adding to tank.
- Activated carbon in the filter will remove tannins.
- Regular water changes will gradually clear tannin staining.
- Consider leaving it — many fish species benefit from slightly tannin-stained water.
Universal Fixes for Cloudy Water
- Test your water first. Use an API Master Test Kit to check ammonia, nitrite, nitrate, and pH before doing anything. This tells you if cloudiness is dangerous (bacterial bloom with ammonia spike) or cosmetic (tannins).
- 30% water change. Dilutes whatever is causing the cloudiness, reduces nutrients feeding algae, and gives your filter fresh substrate. Use dechlorinated water at the same temperature.
- Check your filter. Is it running at full flow? Is the media clogged? Rinse mechanical filter media in old tank water (not tap water) to restore flow without killing bacteria.
- Reduce feeding. Cut back to once daily, only what fish consume in 2 minutes. Uneaten food is a primary driver of bacterial blooms and green water.
- Adjust the light schedule. Reduce to 8 hours max. Move the tank away from direct sunlight if applicable.
- Add fast-growing plants. Hornwort, water wisteria, and floating plants compete aggressively with algae for nutrients. A heavily planted tank rarely gets green water.
⚠️ Avoid Chemical Clarifiers in New Tanks
Products like API Accu-Clear cause particles to clump together so the filter can catch them. They work well in established tanks but can interfere with the bacterial cycling process in new tanks. Wait until your tank is fully cycled before using them, or stick to natural methods.
When Is Cloudy Water Dangerous?
Most cloudy water is cosmetic — a bacterial bloom clears itself, tannins fade, and green water responds to light reduction. Cloudiness becomes dangerous when it’s accompanied by:
- Fish gasping at the surface or swimming erratically
- Elevated ammonia or nitrite on a test kit
- Foul smell (rotten egg = hydrogen sulfide)
- Fish clamping fins or appearing lethargic
If any of these signs are present, do an emergency 50% water change immediately and test your parameters. See our Why Are My Fish Dying guide for full troubleshooting.
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