What causes cloudy water and how to fix it
Cloudy aquarium water is almost always caused by one of three things: a bacterial bloom (the most common), suspended particles from gravel or décor, or algae growth. A bacterial bloom looks like a milky haze and happens when beneficial bacteria multiply rapidly—usually after you set up a new tank, do a large water change, or add too much food. The water clears on its own in a few days as bacteria settle, but you can speed it up by doing a partial water change (25 percent of the tank volume) and reducing feeding to once a day.
If the cloudiness appeared right after you filled the tank or stirred the gravel, you are seeing suspended particles. Let the tank sit undisturbed for 24 hours and the particles will settle to the bottom. If it is still cloudy after a day, or if the water looks greenish or brownish, you have an algae problem or a filter that is not working hard enough. Check that your filter intake is not clogged and that water is flowing through it at the normal rate.
Key Takeaways
- Milky cloudiness usually clears within three to five days on its own as bacteria settle, but a 25 percent water change speeds recovery.
- Particles from new gravel or décor settle within 24 hours if you leave the tank undisturbed and do not stir the substrate.
- Green or brown cloudiness points to algae or a filter problem—check that water is flowing freely through the filter media.
- Overfeeding is the most common cause of bacterial blooms, so reduce feeding to once daily until the water clears.
- Do not do a complete water change or replace all the filter media at once, as this removes the bacteria your tank needs to stay healthy.
The bacterial bloom: why it happens and what to do
A bacterial bloom occurs when the population of heterotrophic bacteria—the kind that eat organic waste—explodes because there is suddenly a lot of food available. This happens most often in new tanks during the first two weeks, when the bacterial colony is still establishing itself and any excess food or fish waste triggers rapid growth. It also happens after you do a large water change (more than 50 percent) or add new fish without letting the filter catch up.
The bloom is not harmful to fish, but it clouds the water and makes it hard to see. To speed recovery, reduce feeding to once a day or skip a day entirely, do a 25 percent water change, and make sure your filter is running. Do not replace the filter media—that removes the bacteria you need. The cloudiness should clear within three to five days. If it lasts longer than a week, the problem is likely overfeeding or a filter that is too weak for your tank size.
Particle cloudiness from gravel and new décor
When you first fill a tank or add new gravel without rinsing it thoroughly, fine dust and particles suspend in the water and make it look cloudy or chalky. This is purely cosmetic and does not harm fish. The particles are too heavy to stay suspended and will settle to the bottom within 24 hours if you do not disturb the water. Avoid using an air stone or powerhead during this time, as they keep particles in suspension.
If you are setting up a new tank, rinse the gravel in a bucket under running water until the water runs clear before adding it to the tank. This removes most of the dust and cuts cloudiness significantly. If you forgot to rinse and the tank is already cloudy, straightforward wait 24 hours without running equipment that creates water movement. You can also do a 25 percent water change to remove some of the particles faster, but waiting alone will work.
Green or brown water: algae and filter problems
If the water is tinted green or brown rather than milky white, you have either an algae bloom or a filter that is not removing waste fast enough. Green water is caused by free-floating algae (not the algae that grows on glass and décor). Brown water usually means tannins from wood or peat are leaching into the tank, or the filter is clogged and waste is building up.
For green water, do a 50 percent water change, clean the filter, and reduce light to 6 to 8 hours per day. Algae needs light and nutrients to grow, so less light and fewer nutrients (from fish waste) will slow it down. For brown water from wood, soak the wood in a bucket of water for a few days before adding it to the tank, or accept that the tint will fade over time as the tannins leach out. If the water is brown and smells bad, your filter is clogged—rinse the filter media in old tank water (not tap water) to restore flow.
When to do a water change and how much
A partial water change helps clear cloudiness by removing some of the particles or excess nutrients that feed bacteria and algae. For a bacterial bloom, a 25 percent change is enough. For green water or particle cloudiness, a 25 to 50 percent change works better. Do not do a complete water change (100 percent) unless the water smells foul or fish are gasping at the surface—a full change removes beneficial bacteria and can shock fish.
When you do a water change, use water that is the same temperature as the tank water. Cold water stresses fish and can trigger more bacterial growth. If you are treating a bloom, do the water change once and then wait three to five days before doing another one. Frequent water changes can actually prolong a bloom by removing some bacteria but leaving enough to keep multiplying. Let the filter do the work between changes.
Filter maintenance to prevent and clear cloudiness
A clogged or undersized filter is often the real cause of persistent cloudiness. Check that water is flowing freely through the filter outlet—if the flow is weak or slow, the filter media is dirty. Rinse the filter media in a bucket of old tank water (water you removed during a water change) to restore flow. Never rinse filter media in tap water, as chlorine and temperature changes kill the beneficial bacteria living in the media.
If your tank has been cloudy for more than a week despite water changes and reduced feeding, your filter may be too small for the number of fish you have. A filter should process the entire tank volume at least three times per hour. If you have a 20-gallon tank, your filter should move at least 60 gallons per hour. Check the filter's flow rate on the box or manual. If it is undersized, consider upgrading to a larger filter or reducing the number of fish.
How long cloudiness takes to clear
Particle cloudiness clears within 24 hours if you leave the tank undisturbed. A bacterial bloom usually clears within three to five days with reduced feeding and normal filtration, or within one to two days if you do a 25 percent water change. Green algae water takes longer—usually five to seven days—because you need to reduce light and nutrients, which takes time to show results.
If cloudiness lasts longer than a week, the cause is likely overfeeding, a filter that is too weak, or a light that is on too long each day. Check all three before assuming there is a deeper problem. Most cloudy water clears on its own with patience and basic maintenance. Avoid the temptation to do multiple large water changes or replace all the filter media at once, as both of these can make the problem worse by removing the bacteria your tank needs.
Frequently Asked Questions
Will cloudy water hurt my fish?
No. Cloudy water from bacteria, particles, or algae does not harm fish. Fish can breathe and eat normally. The only exception is if the water smells foul or fish are gasping at the surface, which means waste is building up faster than the filter can remove it—in that case, do a 50 percent water change when ready.
Can I use a water clarifier product to clear the cloudiness faster?
Water clarifiers work by clumping particles together so they settle faster, but they do not speed up bacterial blooms or algae problems. If your cloudiness is from particles, a clarifier will help. If it is from bacteria or algae, a clarifier will not help and may stress fish. It is safer to wait three to five days and let the tank clear naturally.
Should I do a complete water change to clear the cloudiness?
No. A complete water change removes the beneficial bacteria living in the filter and water, which can make cloudiness worse or cause the tank to cycle again. Do a partial change of 25 to 50 percent instead. The only time to do a complete change is if the water smells rotten or fish are in when ready danger.
Why is my tank still cloudy after two weeks?
If cloudiness lasts longer than a week, check three things: are you feeding too much (reduce to once daily), is the filter clogged (rinse the media in old tank water), and is the light on more than 8 hours per day (reduce to 6 to 8 hours). One of these is usually the cause. If all three are fine, your filter may be undersized for the tank.
Can I add more fish while the water is cloudy?
No. Wait until the water is clear before adding more fish. Adding fish during a bacterial bloom or algae bloom adds more waste and prolongs the cloudiness. Once the water is clear for at least three days, it is safe to add new fish slowly—one or two at a time, with a few days between additions.