Step-by-step
Step 1: Identify the type of cloudyWhite/milky = bacterial bloom (recent feeding + overstocking + uncycled tank). Yellow/tannins = driftwood leaching (cosmetic). Green = algae (lighting issue + nutrients). Cloudy + brown sediment = recently disturbed substrate. White + film on top = excess oils + dissolved organics.
Step 2: Test water parametersCheck ammonia + nitrite + nitrate. Detectable ammonia = bacterial bloom or cycle disruption (do not water-change yet - wait for it to consume the food source). Zero ammonia = different cause.
Step 3: Bacterial bloom (white/milky)Stop feeding for 5 days. Increase filtration if you have spare media. Add pure-strain bacteria supplement (Stability, Bio-Spira). Bloom self-resolves in 5-7 days as bacteria starve out the rapid-bloom species.
Step 4: Tannin yellowingCosmetic - many fish prefer it. Run carbon to remove. Tannins in moderate amounts boost fish coloration + immune systems.
Step 5: Green water (algae)Reduce lighting hours by 2. Add a UV sterilizer (kills suspended algae in the water column). Reduce phosphate (run GFO in saltwater, EI fertilization audit in planted).
Step 6: Sediment from substrateWait 24-48 hours. Sediment settles. If persistent: replace mechanical filter floss; add polishing pad or fine micron filter sock.
Step 7: Dissolved organics filmIncrease surface agitation. Add carbon. Run a protein skimmer (saltwater - sucks DOC out of water column). Reduce feeding.
Frequently asked questions
How long does cloudy water last?
Bacterial bloom: 5-7 days self-resolving. Tannin yellowing: until tannins are removed (could be days, weeks, or never if driftwood keeps leaching). Green water: 1-3 days with UV sterilizer. Sediment: 24-48 hours.
Should I do a water change for cloudy water?
Depends on cause. Bacterial bloom: NO - water changes feed the bloom by adding nitrogen back. Tannin/algae/sediment: yes, water changes help.
Is cloudy water dangerous to fish?
Bacterial blooms are usually harmless (they consume oxygen but at low rates). Algae blooms drop oxygen at night. Test parameters - if ammonia is 0 and oxygen seems normal, fish are fine.