Skip to content
AquaReefBasin

Clarity and color

The water has gone cloudy and white

A white or gray haze that appeared over a day or two in a tank under two months old is almost always a bacterial bloom, and it clears on its own once the biological filter catches up. The important thing is not the cloudiness, it is what your ammonia and nitrite tests say while it is happening. Test both before you do anything else, because the bloom is harmless and a 2 ppm ammonia reading behind it is not.

Act this week

What this problem usually shows in the water

Test these first

Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.

  1. Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
  2. Nitrite — nitrite ion, the intermediate product between ammonia and nitrate
  3. Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number

What is usually behind it

Causes at a glance
CauseHow to confirmTime to resolve
Bacterial bloom in a young or recently disturbed tankThe haze is even throughout the tank rather than settling, it appeared within a day or two, and the tank is under about eight weeks old or has just had its filter media replaced. Ammonia or nitrite is usually detectable at the same time.Usually clears in three to seven days once feeding stops
New substrate that was not rinsed, or was stirred upThe cloudiness arrived immediately after setup, a rescape, or a deep gravel vacuum, and it is finest near the bottom. Held to the light it looks like dust rather than milk, and it settles when the filter is switched off for an hour.Twelve to forty-eight hours with mechanical filtration running
Overfeeding, or a dead fish or snail somewhere out of sightAmmonia is up out of proportion to the age of the tank, there is often a smell, and the food count does not match the fish count. Look behind the filter, under decor and in the intake strainer.Water clears within two to four days after the source is removed
A clarifier or conditioner reacting with what is already dissolvedThe haze appeared within an hour of adding something to the tank, and nothing else changed. Some clarifiers deliberately clump fine particles, which looks worse before it looks better.Twenty-four hours, or one water change

Bacterial bloom in a young or recently disturbed tank

How to confirmThe haze is even throughout the tank rather than settling, it appeared within a day or two, and the tank is under about eight weeks old or has just had its filter media replaced. Ammonia or nitrite is usually detectable at the same time.

What to doChange 25% of the water to bring the ammonia reading down, stop feeding for two days, and then leave it alone. The bloom is free-floating bacteria feeding on dissolved organics; it starves once the filter colony is large enough to take the food away from it.

Time to resolve: Usually clears in three to seven days once feeding stops

New substrate that was not rinsed, or was stirred up

How to confirmThe cloudiness arrived immediately after setup, a rescape, or a deep gravel vacuum, and it is finest near the bottom. Held to the light it looks like dust rather than milk, and it settles when the filter is switched off for an hour.

What to doLet the filter run and give it time. If it is very heavy, take the mechanical media out, rinse it in a bucket of tank water, and put it back — sediment clogs floss quickly and the filter stops clearing anything once it does.

Time to resolve: Twelve to forty-eight hours with mechanical filtration running

Overfeeding, or a dead fish or snail somewhere out of sight

How to confirmAmmonia is up out of proportion to the age of the tank, there is often a smell, and the food count does not match the fish count. Look behind the filter, under decor and in the intake strainer.

What to doRemove whatever is decaying, then change 30% of the water and vacuum the substrate while you do it. Cut the feeding to what disappears in about a minute, once a day.

Time to resolve: Water clears within two to four days after the source is removed

A clarifier or conditioner reacting with what is already dissolved

How to confirmThe haze appeared within an hour of adding something to the tank, and nothing else changed. Some clarifiers deliberately clump fine particles, which looks worse before it looks better.

What to doWait a day. If it has not settled, change 25% of the water and let the mechanical media catch the clumps. Do not add a second dose on top of the first.

Time to resolve: Twenty-four hours, or one water change

What not to do

These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.

  • Do not change all the water. A full change strips the buffer and the dissolved chemistry the tank has settled into, and it does not remove the cause.
  • Do not replace the filter media because the water looks dirty. The media is where the bacteria that end this live.
  • Do not add a clarifier and a bacteria product and a conditioner in the same session. If it improves you will not know why, and if it worsens you will not know what to stop.
  • Do not keep feeding at the normal rate to compensate. Feeding is the fuel supply for the bloom.

When this stops being a water question

If fish are gasping at the surface, hanging in the filter outflow, or sitting on the bottom with clamped fins while this is happening, treat that as a water emergency first — test and change water now. But an animal that stays unwell after the water is right is a medical question, and the person to ask is an aquatic veterinarian, not this site.

Keeping it from coming back

Blooms are a young-tank event, so most prevention is really about not restarting the clock. Stagger filter media changes so an established piece always stays in the filter, rinse media in old tank water rather than under the tap, and add fish in small groups with a couple of weeks between them. Rinse new substrate until the runoff is clear before it goes in, and vacuum in sections rather than turning the whole bed over in one session.

Reference for this page

Equipment involved

Related problems

Published sources

Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.