Why Is My Fish Tank Cloudy? Causes and Fixes

Start with a white cup

One retailer suggests pouring some tank water into a plastic cup or bucket that is solid white inside, so you can see its true color. Then match what you see:

What you see	Most likely cause	What to do
Like diluted milk, no visible specks	Bacterial bloom	Test ammonia. If it reads zero, wait
Specks or particles	Waste, food or substrate dust	Clean and rinse, and give the filter time
Green tint, like pea soup	Algae	See our green water guide
Brown tinge	Tannins from wood or leaves	Usually cosmetic, so change water or use carbon
Nothing in the cup	The glass, not the water	Scrub the panes, and check for glare

Check these first

Whatever you see in the cup, check two things before you change anything. One aquarium service owner quoted by a retailer says your first tasks are to check the temperature and make sure ammonia is zero.

Cloudy water does not usually harm fish on its own, but the cause might. A fish veterinarian lists cloudy, foul-smelling water among the signs of new tank syndrome, alongside lethargy and decreased appetite. If ammonia or nitrite is showing, deal with that first.

Milky water: a bacterial bloom

When there are more nutrients in the water than the helpful bacteria can use, bacteria multiply quickly and the water turns hazy. One keeper site explains that the haze is often more than bacteria, since tiny creatures soon move in to feed on them.

Blooms are most common in these situations:

• A new tank. One retailer’s guide says you should expect one during cycling.
• A big group of fish added at once. The bacteria cannot keep up yet.
• Too much cleaning. Over-cleaning, large repeated water changes or replacing filter media can remove the bacteria.
• Overfeeding. One service owner says overfeeding is very often at the root of the problem.
• Some medications. Antibiotics in particular can kill helpful bacteria.

The fix is to wait. One retailer says to wait one to two weeks and warns that lots of water changes or a UV sterilizer just make a bloom last longer. Another quotes an expert: if there is a bloom and no ammonia, do nothing. A keeper site says beginners often panic at this stage.

Specks: dust, waste and food

• New substrate. Tiny bits float up when you set up a tank or plant it. One retailer says the powder usually settles or reaches the filter within a few days. If it lasts more than a week, it suggests large water changes, or rinsing the substrate. Rinsing gravel before it goes in avoids this.
• Built-up waste. Vacuum the substrate on a regular schedule, and clean filter media in old tank water.
• Messy food. One retailer suggests foods that are eaten quickly and leave firmer waste.
• Bottom feeders. Fish that dig stir up the floor. More mechanical filtration can help.

Brown water and hazy glass

Brown water usually comes from tannins, natural compounds from driftwood and leaves. One retailer says water changes remove them gradually, soaking new wood first helps, and activated carbon can clear what remains.

If the cup looks clear, the haze is on the glass. Scrub the inside, and clean the outside with an aquarium-safe cleaner.

Do not do this

• Do not change all the water. One service owner says never to change 100%, or to deep clean the tank and accessories.
• Do not change everything at once. After fixing the cause, give the bacteria time to rebalance.
• Do not skip the test. Clear water can still hold ammonia, and cloudy water may hold none. A test kit is the only way to know.

If your fish look sick, ask a vet or fish specialist. One service owner warns that some treatable diseases spread very quickly.