The short answer
Cloudy water is almost always one of two harmless things: a bacterial bloom (a milky white haze, common in new tanks) or fine debris stirred up from the substrate. Neither is dangerous to fish on its own. The cure is patience, light feeding and small regular water changes — not draining and refilling the whole tank, which only restarts the cycle.
Milky white haze: a bacterial bloom
A white or greyish cloudiness in a tank set up in the last few weeks is a bloom of free-floating bacteria feeding on excess nutrients. It typically appears while a tank is still cycling and clears by itself as those bacteria colonise the filter and surfaces. Don’t overreact by scrubbing everything or replacing all the water — that removes the very bacteria you want to establish.
Feeding less is the biggest lever. Uneaten food and heavy waste give the bloom fuel, so scale back portions and remove any leftovers. If your tank is new, read how to cycle an aquarium so the process finishes cleanly, and consider a bacteria starter to speed things along.
Grey or brown haze: fine debris
If the cloud looks like suspended dust rather than milk, it’s usually fine particles: uncleaned new substrate, disturbed detritus, or floating waste. Rinse new gravel or sand before adding it, and vacuum the substrate during maintenance. Good mechanical filtration traps the rest — see our filter guides and use a gravel cleaner to lift muck out during a water change.
What not to do
Don’t overfeed, don’t over-clean the filter, and don’t dose “clearing” chemicals as a first move. Address the cause — food, debris or a new-tank cycle — and the water clears on its own. If you’re unsure what’s going on, a liquid test kit tells you whether it’s a cosmetic haze or a real water-quality problem worth acting on.