Every aquarium owner wants healthy fish, and for healthy fish, you need clean and clear water. At some point in this hobby, you will have to deal with a cloudy fish tank. Do not worry, we have all been there and are here to help.

Cloudy fish tank water is caused by extra particulate matter or bacteria in the water. It can be dealt with via partial water changes (except in a new aquarium! – more on that later), chemicals, better filtration, or changes in the food quality and quantity used.

Causes of Cloudy Water

Cloudy water can have multiple causes. And some of these causes are more of a problem than others.

In a New Tank

A new tank is biologically a clean slate, and the populations of healthy bacteria needed to maintain water quality have not yet been built up. Every other microbe in the neighborhood is there pigging out on the buffet of new nutrients. The haze present is a bacterial bloom and is not necessarily bad. The tank will clear up in a couple of weeks as the bacteria run their course in a process called the nitrogen cycle. 

Cloudy water in a new tank also can be caused by sedimentary dust from new gravel, or by high concentrations of dissolved minerals in your tap water. All of these causes will give your water a milky gray appearance. 

In an Established Tank

Established tanks can be prone to cloudy green water, and this is one time that going green is a bad thing. Green water indicates out-of-control algae in your tank, usually caused by too much light, excess nutrients like phosphates from rotting food, or nitrates from fish waste.

Sometimes fish keepers experience a yellow or brownish haze to their water. This condition is rare and is typically caused by the introduction of organic matter from the wild, such as driftwood or leaf litter. The tannins in the leaves or wood leach out into the tank, tinting the water yellowish. Called black water, it can be unsightly, but it is not harmful to your fish. You can prevent leaching in your tank by pre-soaking any wild woody product in clean water for a few days to a week.  

Is Cloudy Water Bad for My Fish?

Cloudy water is not bad for fish in most cases. The cloudiness caused by a bacterial bloom during the nitrogen cycle is nothing to worry about, provided that it clears up in a couple of weeks. The same goes for sediment-caused haziness. 

Cloudiness caused by algae can be cause for concern as they are feeding on excess nitrates. Nitrites are a result of high levels of ammonia (fish waste) in the water. High levels of ammonia can cause chemical burns on fish fins and gills, leading to respiratory distress.

Fixing Cloudy Water

Gray Water

Cloudy water can be easy to fix. The most common method is to perform a partial change of the water to clear out some of the affected H2O and replace it with clean. A water change is the fastest way to remove excess phosphates and nitrates from your tank. It is important that you do not perform partial water changes on a new tank as this will disrupt the nitrogen cycle, and you will perpetually battle a cloudy tank until a cycle is complete.

If you have stubborn particulate matter that does not clear up with water changes or vacuuming, then you may need to add a water clarifier. These are known as flocculants. They cause debris to clump up so that it can be caught in the filter media.

Green Water

Battling the algae that cause green cloudy water can be a longer fight than clearing gray water but is still very winnable. First, reduce the number of hours per day that the aquarium is lighted. Algae rely on photosynthesis to eat, so cut down the light to starve them out. 

Next, test your water for phosphates and nitrates, and use the appropriate chemicals to bring the levels into check. If needed, switch to using water filtered by reverse osmosis to lower the amount of phosphate present in the tank. 

Vacuum the gravel on the bottom of the tank to remove built-up fish waste and leftover food. If left to rot, these will contribute to rapidly rising phosphate and nitrate levels. Vacuuming the tank will also lower the water level, allowing for a partial water change. 

Preventing Cloudy Water

Top Five Tips for Clear Water 

  1. Weekly water changes. These should be just a little over ten percent of the tank’s volume. Water changes will help to remove phosphates and nitrates from the tank and bring in some oxygen.
  2. Vacuum regularly. Keeping your gravel free of waste and decaying matter will prevent the build-up of excess phosphates and nitrates like ammonia. 
  3. Pre-wash new gravel. New tank gravel carries a lot of dust, and you do not want that in your tank. Put the gravel in a bucket or strainer and rinse it till the water runs clear.
  4. Change your chemical filter often and your mechanical filter when needed. Carbon filters are very effective at removing impurities in the water but should be changed monthly or when they become saturated with impurities, whichever comes first. A mechanical filter, such as a sponge, can work as a pre-filter to a chemical filter. 
  5. Sponges should only be cleaned in aquarium water, if possible, to maintain the good bacteria present. An ideal setup is to run two sponges, one behind the other. The front sponged is pulled to clean and then inserted back behind the remaining sponge. Swapping filters on a six-week cycle will allow the back sponge to build a colony of nitrite-eating bacteria before the front sponge is removed to clean. 

Five More Tips for Clear Tank Water

  1. Aerate your tank well. The bacteria that break down the nitrites need plenty of oxygen to do their job.
  2. Do not overpopulate the tank. Having too many fish in the tank, especially heavy feeders, will cause the nitrate level in your tank to rise quickly. The beneficial bacteria in the filter system will become overwhelmed, and the tank will require excessive water changes and/or a larger filter system to keep the ammonia at safe levels for the fish.
  3. Do not overfeed. Extra food just makes more debris on the bottom of the tank. Unless you really like vacuuming your fish tank, of course, then proceed with the excess feed.
  4. Use a good flake food. Flake foods should not cloud water like some of the high-protein offerings like cichlid pellets or mussel. Look for a variety that mentions low waste or clear water.  
  5. Test your water regularly. The ammonia and nitrate levels should be nil. Stock the chemicals needed to keep your PH in check and to reduce the ammonia level in an emergency, and use them promptly when needed.


A cloudy fish tank can be prevented, with prior planning and regular maintenance. The appearance of bacteria or algae in your tank may be alarming, but it is not the end of the world and it can be rectified with a little effort. If you do your part for prevention, your tank should stay crystal clear for long periods of time. Enjoy!

Brandon Tanis