Cloudy aquarium water: 6 causes and fixes

"The water clouded up" is a catch-all complaint with several different problems behind it. The color and character of the cloudiness point to the cause. The fix is to remove the cause, not to dose "clarity chemicals."

1. Milky white cloud — bacterial bloom

Happens in the first 2–3 weeks after setup or after a massive water change/cleaning. Heterotrophic bacteria multiply on organics faster than nitrifiers can process them.

Fix: do nothing. The cloud clears on its own in 3–7 days. Don't change water — that only prolongs the problem. Feed minimally.

2. Green cloud — algae "water"

Microscopic green algae bloom throughout the water column due to excess light and nutrients.

Fixes:

• UV sterilizer for a week — kills algae in the water.

• "Blackout": 3 days of total darkness (dense cloth over the tank).

• Cut photoperiod to 6 hours for 2 weeks.

• Reduce fish feeding (excess phosphate feeds the algae).

3. Yellowish but clear — tannins

Fresh driftwood or oak leaves release tannins — humic acids. Stain water amber. This is a natural "Amazon biotope" look that doesn't affect fish health.

If you don't like it aesthetically: activated carbon in the filter for 1–2 weeks.

4. Gray cloud with sediment — disturbed substrate

After siphoning or moving driftwood. Usually settles in a few hours. If it doesn't settle — substrate grain is too fine. Replace with coarser material.

5. Persistent slight haze — overstocking and overfeeding

The biofilter can't keep up with the biomass. The fix is systemic:

• Reduce stocking or upsize the tank.

• Cut food portions by 30 %.

• Add filter capacity.

• Change water more often (50 % once a week).

6. Rainbow film on the surface — biofilm

Not cloudiness but an oily film of bacteria and lipids. Plants, food, skin oils on hands, room aerosols — typical sources.

Fix: skim the film with a paper towel; aim a spray bar at the surface; add aeration that breaks up the film.

Diagnostic algorithm

1. First, quick test: NH₃, NO₂, NO₃. If NH₃ or NO₂ is above 0 — the cloudiness is tied to a broken biofilter.

2. Then visual inspection: color, sediment, film.

3. By the symptoms — pick one of the 6 causes above.

4. Treat the cause, not the symptom. No "cloudiness cures" — they're either placebo or one-time effects.