Cloudy water in a goldfish tank

“My tank has gone cloudy” describes at least five different situations with different causes, different urgencies and opposite responses. Fortunately they are easy to tell apart, because the colour and the timing between them carry nearly all the information you need.

The one worth understanding properly is the first: a white bacterial bloom in a young tank, which is a common one and the one where the instinctive response tends to make matters worse.

Which one do you have

Which cloudy water you have

1. Milky white

   A bacterial bloom. In a new tank the haze usually clears by itself; keep testing ammonia and nitrite.

2. Green

   Free-floating algae: a different problem.

3. Tea-coloured, clear

   Tannins from wood. Harmless.

4. Brown, opaque

   Stirred substrate. It settles.

5. Settles overnight

   Dust from unrinsed sand. Not biological.

What you see	Usually	What to do
Milky white or grey haze, new tank	Bacterial bloom	Leave it; do not clean the filter
Milky white haze, established tank	Bloom triggered by something — find it	Investigate, then leave it
Green, from a faint tint to pea soup	Free-floating algae	A different problem entirely
Yellow or brown tint, water still clear	Tannins from wood	Harmless; carbon if you dislike it
Brown and genuinely opaque	Stirred substrate, or a diatom film in a young tank	Stirred substrate settles; vacuum
Fine white haze that settles overnight	Substrate dust from unrinsed sand or gravel	Settles; it is not biological
Any cloudiness with a bad smell	Something is decomposing	Find it today

Two of those rows point elsewhere. Green water is covered on the algae page, where it belongs, because it is an algal bloom with its own causes and its own fix. And a smell changes the priority entirely, which the last section returns to.

The new-tank bloom

Set up a new aquarium, fill it, and within three to seven days the water often goes milky. It happens to many new tanks, so it is worth knowing about before it happens to yours.

What it is. The usual explanation is a population explosion of heterotrophic bacteria — organisms that feed on dissolved organic carbon rather than on ammonia. A new tank has plenty of that carbon and few established surfaces where bacteria live, so the bacteria live suspended in the water instead. They are individually invisible, but in the billions they scatter light, and the water turns white.

Why it ends. It is thought to be self-limiting. The bacteria consume their food supply faster than it is replaced, the population crashes, and the survivors settle onto surfaces — the substrate, the glass, the filter media — where bacteria are supposed to live. Keepers report that it lasts from a few days to a couple of weeks; there is no published timing.

What it is not. It is not an algae problem, it is not dirt, and it is not evidence that you have done something wrong. Nor is it the same thing as cycling, which is the confusion the next panel addresses.

Why the obvious responses make it worse

The instinct on seeing cloudy water is to clean things. Each part of that instinct tends to be counterproductive here.

Cleaning the filter removes the surfaces the suspended bacteria are trying to colonise, and in a young tank it removes the nitrifying population you are waiting for. A bloom is a poor moment to rinse media.

Large water changes remove some suspended bacteria, but the dissolved organics that feed them are replaced continuously by the fish, and the haze can return quickly. You have swapped a lot of water for a short spell of clarity, and in a cycling tank you have also diluted the ammonia the nitrifiers need to establish.

Deep-cleaning the substrate releases trapped organic matter into the water, which is food, and can extend the bloom.

Clarifiers clump the particles so the filter can catch them. That clears the water without touching the cause, loads the media quickly, and in a heavily stocked tank removes a large bacterial mass suddenly — see the oxygen section below.

Stopping feeding entirely is the one instinct that tends to help, and a day or two of it is reasonable. Feeding less is the one intervention aimed at the cause, because uneaten food is a rich carbon source.

The part that actually matters for goldfish

Cloudiness is cosmetic. Oxygen is not.

A heavy bacterial bloom is a very large number of respiring organisms in the water, and they use dissolved oxygen as they go. In a lightly stocked tank that is rarely a problem. In a goldfish tank it can be, especially in one that is already near its stocking limit: goldfish tolerate low oxygen better than many aquarium fish, but at room temperature that means hours, and a warm, crowded tank with a bloom can still run short. The anatomy page covers how goldfish breathe.

The risk is often greatest in the early hours of the morning, after a full night of respiration with no photosynthesis to offset it. If fish are hanging at the surface, gathering at the filter outflow, or breathing hard during a bloom, that is an oxygen problem rather than a cloudiness problem, and the response is immediate aeration: an air stone, a lowered water level to increase surface agitation, a second outflow angled at the surface. The aeration guide covers the equipment.

Blooms in an established tank

A tank that has been clear for months and goes cloudy is telling you something changed. Unlike the new-tank version, this one is worth investigating rather than waiting out.

Common triggers:

• A period of overfeeding, or a feeding while you were away that was more generous than intended.
• Something dead and unfound — a fish behind the filter, a snail in the substrate. This is also the version that smells.
• A deep substrate clean that released months of trapped organics at once.
• A filter rinsed too thoroughly, so the mechanical stage stopped catching particles while the biological stage also took a hit.
• A new fish or several, raising the organic load faster than the established bacteria can absorb.
• A treatment or medication, several of which disrupt bacterial populations and produce a bloom as they settle.

In each case the bloom is a symptom and the trigger is the thing to address. Test the water while you are at it: a bloom in an established tank occasionally accompanies an ammonia spike, and that is the reading that changes what you do.

The versions that are not bacteria

Tannins. Driftwood leaches tannins for months, tinting the water yellow or brown. The water stays clear — you can see straight through it, it is simply coloured. It is harmless, slightly acidic, and a matter of taste. Activated carbon removes it within days, and pre-soaking new wood in a bucket for a few weeks removes most of it before it reaches the tank.

Substrate dust. New sand or gravel that was not rinsed thoroughly produces a fine haze that settles within a day and returns whenever the substrate is disturbed. It is inert, it is not biological, and the fix is mechanical: let it settle, vacuum it out, rinse better next time. The substrate guide covers preparation.

Diatoms. A young tank often passes through a brown dusty phase as diatoms colonise surfaces, and disturbing them puts brown particles into the water. It resolves as the tank matures.

Post-medication haze. Several treatments cloud the water directly or kill enough bacteria to trigger a bloom afterwards. Expected, temporary, and a reason to keep aeration up during any course of treatment.

The chemical one people miss

Not every white cloud is alive.

Mixing waters of different chemistry can precipitate dissolved minerals out of solution as a fine white suspension — most often calcium carbonate. It happens when a buffer is added too fast, when crushed coral or a carbonate additive goes into soft water in quantity, when very hard tap water meets a tank at a different temperature, and occasionally after a large water change in a hard-water area.

The tells are distinctive. It appears within minutes of doing something, rather than over days. It is a flat, even, chalky white rather than the slightly translucent haze of bacteria. And it often settles as a fine dust on surfaces rather than staying suspended.

It is harmless in itself, but it is worth recognising for two reasons. The first is that it is not a bloom, so waiting for it to “clear on its own” through biological means is the wrong model — it settles or dissolves as the chemistry equilibrates. The second is that it is a signal: something moved your water chemistry quickly, and rapid chemistry movement is one of the few things goldfish genuinely tolerate poorly. The pH, KH and GH page covers doing that slowly.

What to expect, day by day

For the new-tank case, which is the one most people meet, the sequence is usually predictable enough to plan around, though the timings are what keepers report rather than measured figures.

When	What is happening	What you see
Days 1–2	Substrate dust settling, water equilibrating	Haze that clears as it settles
Days 3–7	Heterotrophs multiplying on abundant dissolved carbon	Milky white, often appearing overnight
Days 7–14	Food supply exhausted, bacteria settling onto surfaces	Gradual clearing from the top down
Weeks 2–8	Nitrifiers establishing, entirely invisibly	Clear water, ammonia and nitrite still moving
Any time after	A bloom means something changed	Investigate rather than wait

The row that matters most is the fourth, because it is the one people misread. Clear water at two weeks suggests the bloom is over. It says nothing whatever about whether the tank can process ammonia — and adding a full stock of fish at that point, on the strength of how the water looks, is a common way for goldfish to be lost. The cycling guide sets out what actually indicates readiness.

Things that genuinely speed it up

If patience is not available, keepers report that a few interventions help.

Feed less, or not at all for a day or two. Uneaten food and the waste from a generous feed are rich carbon sources in the tank. This is the one lever here that works on the cause rather than the symptom.

Add mature filter media from an established tank, or a sponge that has been running elsewhere. This gives the suspended bacteria somewhere to go and accelerates the nitrifiers at the same time — a useful thing for anyone with a second tank to lend you. The filtration guide covers what carries the bacteria and what does not.

Add fine mechanical filtration. A pad of filter floss or a polishing pad in the filter catches particles the coarser media passes, and it does visibly clear particulate haze. It does less for a bacterial bloom, because free bacteria are smaller than the pores, but it handles substrate dust and diatoms well. Rinse or replace it often, since a clogged polishing pad restricts flow.

A UV clarifier damages suspended algae as water passes through it, so they clump and are filtered out, and it is mainly bought for green water; whether it helps with a bacterial bloom is not established. It is an effective piece of equipment and an unnecessary one for a problem that often resolves itself; it earns its place in a pond or on a tank with a recurring green-water problem rather than on a new aquarium.

The reason to care beyond appearance

There is a welfare argument for clearing cloudy water that has nothing to do with how the tank looks.

Almost every early warning a goldfish gives is visual. Clamped fins, a fin held slightly tighter than yesterday, a belly a little fuller, a fish hanging back at feeding time, the first grains of white spot — all of it is information you get by looking, and all of it is unavailable through milky water. The healthy baseline page argues for a weekly two-minute look at each fish; a fortnight of cloudy water is a fortnight of those looks not happening.

That is not a reason to reach for a clarifier, which fixes the view without fixing anything else. It is a reason to keep feeding light and to check the fish more deliberately than usual while the water is opaque — and, if a bloom in an established tank is persisting, to treat the loss of visibility as one more argument for finding the cause rather than waiting.

When cloudiness is urgent

One combination changes the priority: cloudy water with a smell.

A healthy aquarium smells faintly earthy or of nothing at all. A tank that smells rotten, sulphurous or sour has something decomposing in it in quantity — often a dead fish that has gone unnoticed behind the filter or under decor, sometimes a large pocket of anaerobic substrate that has been disturbed.

That is not a bloom to wait out. Find the source and remove it, then change water. Decomposition on that scale produces ammonia directly and consumes oxygen while it does so, which is the genuinely dangerous version of everything on this page.

Test at the same time. Cloudiness plus a smell plus an ammonia reading is a tank in trouble, and the response is the one from the water change guide: remove the cause, change a good proportion of the water, and keep changing it until the readings are right.

Preventing it

You often cannot prevent the new-tank bloom, and there is no reason to want to — it is part of a tank establishing itself, and a keeper who waits it out learns something useful about patience that fishkeeping will ask for repeatedly.

The later ones are preventable, and by the same handful of habits that prevent most things on this site: feed conservatively, so there is less dissolved carbon to feed a bloom; rinse filter media in tank water and never all at once, so the mechanical and biological stages both keep working; vacuum the substrate in sections over several water changes rather than deep-cleaning the lot; rinse new substrate until the water runs clear; and count your fish occasionally, because the most unpleasant version of this page begins with one that is missing.