A bacterial bloom, not a dust problem
Milky white water that seems to glow under the light is almost always a bacterial bloom: vast numbers of free-floating bacteria feeding on dissolved waste in the water column. These are not the settled, useful bacteria coating your filter media. They are opportunists multiplying in open water because food for them, ammonia and other organics, suddenly became plentiful. The cloud itself rarely harms fish directly. What travels with it, the waste spike feeding the bloom, is the part that can.
Why blooms happen in new tanks
A new aquarium's filter starts with almost no bacterial colony, so waste from the first fish, the first feedings, and even nutrients leaching from new substrate build up faster than the colony grows. The water clouds, clears as the bacteria colonize surfaces, and may cloud again as stocking increases in steps. This is the visible middle of the cycling process, and it is why new tanks should be stocked slowly: each small addition lets the colony resize before the next arrives. A bloom in the first weeks, with tests watched closely, is a phase to manage rather than a failure.
Why an established tank suddenly blooms
In a mature tank, a fresh bloom means something disturbed the balance. Overfeeding is the frequent trigger, including the well-meaning holiday feeder who doubles rations. A thorough clean can cause it too: replacing all the filter media at once, or rinsing media under chlorinated tap water, removes the colony, and the bloom is the replacement crew assembling in the open water instead. A dead fish decaying unseen, a clogged filter cutting flow, or a big new stocking list can each produce the same whiteout. The cloud is a symptom with several possible authors, so read the tank's recent history before reacting.
What to do while the water is white
Test ammonia and nitrite, since those numbers decide your urgency. Keep the filter running continuously, feed sparingly once a day or every other day, and resist large daily water changes, which remove the very bacteria you are waiting on and prolong the cloud. Modest partial changes are reasonable if ammonia climbs, because protecting fish outranks shortening the bloom. Add surface agitation or an air stone if fish gather at the top, since dense bacterial growth consumes oxygen. If fish are gasping, dying, or ammonia stays high despite light feeding and partial changes, contact an aquatic vet or experienced store, because a bloom riding on a genuine cycle crash needs more than patience.
Let the episode teach the tank's rhythm
Most blooms clear within days to a couple of weeks as the bacteria settle. When yours does, the useful question is what fed it, because the answer is usually a habit worth adjusting: smaller feedings, staggered media rinses, slower stocking. Recording the dates, test readings, and the likely trigger in KinsLe's Pet Passport turns a murky week into a reference, so if the water whites out again you can compare episodes instead of starting the detective work from zero.