Troubleshooting: Water and chemistry

Why is my reef tank water cloudy?

The water looks milky, white, grey or hazy instead of clear. It may have come on over a day or two, or in minutes after you added something.

Last checked 21 October 2026. How we check

What should I check first?

1. When did it start? Note anything added in the last 48 hours: new sand, a bacteria product, a carbon dose, alkalinity or kalkwasser, a new salt batch.
2. Test ammonia, alkalinity, calcium and pH.
3. Look at the fish: rapid breathing or gasping at the surface means act now, not later.

What is most likely causing it?

Most likely first. The order is our reading of how often the sources link each cause to this symptom.

1. A bacterial bloom: free-floating bacteria multiplying on spare food or carbon. Milky white haze, usually in a tank under about two months old, or after dosing bacteria or a carbon source such as vodka. Keepers report blooms lasting from a week to two months (forum reports, typical, check).
2. Calcium carbonate falling out of the water (precipitation), often called a snowstorm. White cloud or flakes within minutes of dosing alkalinity, kalkwasser or a new salt mix, often near where the dose went in. Alkalinity and calcium both drop on the next test.
3. Fine dust from new sand. Grey or white haze straight after filling or moving sand, worse where flow hits the sand bed. Clears as it settles or is filtered out.
4. Something rotting. Haze with a bad smell, a missing fish or snail, or ammonia above zero.

What should I do?

1. If you were dosing when it started, stop dosing until you have tested.
2. For a snowstorm: test alkalinity, calcium and magnesium; dose more slowly next time and only into strong flow. Red Sea lists low magnesium and overdosing among the causes of precipitation.
3. For a bacterial bloom: feed less, keep the skimmer running, use fine filter floss or a sock and change it often, and give it time. Do not add more bacteria or carbon in the hope of fixing it.
4. For sand dust: turn powerheads away from the sand for a day and run fine floss until it clears.
5. If ammonia is above zero, look for and remove anything dead, then do a water change with matched salt water.

What not to do

• Chasing a bacterial bloom with chemical clarifiers or big repeated water changes: keepers say water changes rarely clear it on their own (typical, check).
• Adding kalkwasser or alkalinity in one large dose.

When should I get help?

Something dying right now? Go to the emergency playbooks.

Where do sources disagree?