Cloudy aquarium water is one of the most common problems beginners face, and the good news is it's almost always fixable. Here's how to diagnose what type of cloudiness you have and fix it correctly.
Type 1: Milky White Cloudiness (First Week of Setup)
White cloudiness in a brand new tank is almost always a bacterial bloom, a massive population explosion of free-floating bacteria. This is completely normal and is actually a sign that your nitrogen cycle is beginning.
What's happening
Heterotrophic bacteria are multiplying rapidly before the beneficial nitrifying bacteria establish themselves in your filter. They're harmless to fish but make the water look awful.
How to fix it
- Do nothing; this typically clears on its own within 3–10 days
- Make sure your filter is running at full flow
- Do NOT do a large water change; this removes the bacteria and resets the process
- Do NOT add any water clarifiers in the first two weeks
- Test your water every 2 days and monitor ammonia levels
💡 If the white cloudiness appears after you've added fish and doesn't clear within 2 weeks, test ammonia immediately. Persistent cloudiness often indicates ammonia spikes from overfeeding or overstocking.
Type 2: Green Water
Green water is caused by single-celled algae called phytoplankton. It's not directly harmful to fish, but it means your tank has an imbalance that needs fixing.
Common causes
- Too much light, such as a tank in direct sunlight or lighting left on 10+ hours per day
- Too many nutrients, from overfeeding, overstocking, or infrequent water changes
- No live plants competing for nutrients
How to fix it
- Move tank away from direct sunlight immediately
- Reduce lighting to 6–8 hours per day using a timer
- Perform a 3-day blackout: cover the tank completely for 72 hours. Algae cannot survive this, but fish can
- After the blackout, add fast-growing plants (hornwort, water sprite) to compete with algae
- Consider a UV steriliser, the most effective permanent solution for green water
Type 3: Brown or Yellow Tinting
A brown or tea-coloured tint usually comes from tannins leaching out of driftwood. This is not harmful; many fish from South American rivers actually prefer tannin-rich water.
How to fix it
- Pre-soak new driftwood for 1–2 weeks in a bucket, changing water daily
- Boil smaller pieces of driftwood for 1–2 hours
- Add activated carbon to your filter; it will clear tannin discolouration within 24–48 hours
- Replace carbon every 4 weeks as it becomes exhausted
Type 4: Persistent Cloudiness After Cycling
If your tank has been running for several weeks and water remains cloudy, the problem is almost certainly water quality rather than a bacterial bloom.
Diagnostic checklist
- Test ammonia, nitrite, and nitrate; elevated ammonia is the most common cause
- Check if you're overfeeding, and remove any uneaten food after 2 minutes
- Check for dead fish or decaying plant matter
- Perform a 30% water change and re-test after 24 hours
- Check your filter; if clogged, clean media in old tank water (never tap water)
When to Use Water Clarifiers
Liquid clarifiers clump fine particles together so your filter can catch them. They're a temporary fix, not a cure. Use them only after you've addressed the underlying cause. Never use clarifiers in the first 4 weeks of a new tank setup.
Find UV sterilisers, activated carbon, and water testing kits in our Water Care range.
