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All About Cloudy Aquarium Water

Cloudy Aquarium

When trying to enjoy your aquarium there can be nothing more distracting than a cloudy tank. But why did this happen and what caused it to occur? Unfortunately, it might not be a straightforward answer and could be driven by several things. Most of these contributors are easy to rectify but a few may take a little more persistence to resolve. We first need to determine if the water has a white/gray, brown, or green appearance in order to apply the appropriate solution to rectify the discoloration.  

Why is my fish tank cloudy?

If this is a new aquarium setup, one reason for a cloudy appearance could be the substrate you are using. In freshwater aquariums, although most substrates are epoxy coated, all substrates need to be thoroughly rinsed. Substrates for marine aquariums can be extremely dusty and if not thoroughly rinsed, can cause cloudy water. A best practice is to rinse the substrate in smaller quantities to provide a thorough rinsing. If substrate rinsing is skipped, the combination of dust and water can coat the aquarium glass, giving the appearance of a foggy tank.

Cloudy water can also be caused by the water being used. Tap water can contain items like heavy metals, nitrates, phosphates, and silicates. Ensure that you are utilizing a tap water conditioner as many conditioners will neutralize items like chlorine and chloramine, ammonia, and heavy metals. There is also the option of purchasing prepackaged aquarium water or you can choose to purchase a reverse osmosis system. RO filters will help to remove most contaminates from your tap water supply.

After starting a new aquarium, it is not uncommon for the aquarium to become cloudy. This is due to beneficial, nitrogen converting bacteria colonizing to oxidize ammonia and nitrites. This bacteria bloom can also occur in an established aquarium if there is a sudden increase in nutrients. The increased nutrients spark a growth of heterotrophic bacteria. These bacteria break down fish waste, decaying plant debris, and unconsumed food into ammonia. They also create a slimy bio film that will form on decorations and walls of the aquarium. Because the heterotrophs multiply quickly, their explosion in population can cause the water to become cloudy. For a new aquarium with no aquatic life, allow the aquarium to progress through this cycle naturally. In a mature aquarium, reducing the amount of food, completing a water change and cleaning your substrate will help to lower the nutrients in the tank. The addition of live plants can also assist with lowering nutrient levels.

Brown and green discoloration

Brown water certainly takes away from the beauty of your aquatic life and could have been innocently caused by the addition of a popular aquatic décor- driftwood.  If the driftwood was not presoaked prior to adding to the tank can cause a slight brown coloration to the aquarium. This coloration comes from the release tannic acid. This will start clear up with regular water changes and using chemical filtration, like carbon will aid in clarify the water. If driftwood has been added and you are noticing some brown coloration you should test your pH level since tannic acids can lower pH.

Algae bloom in fish tank

Algae blooms in fish tanks

And this brings us to green water in fish tanks… Of all the causes of cloudy/discolored water, green water is the tougher one to rectify. Green water is a bloom of algae cells called Euglena. These planktonic algae cells contain flagella giving them mobility, allowing them to swim closer to the light for photosynthesis. These organisms are both auto- and heterotrophic. Autotrophs can produce their own foods whereas heterotrophs cannot. These algae are always present but will only bloom out of control if something disrupts the equilibrium in the aquarium. Overstocking aquatic life, too much light, high phosphates and excessive nutrients can cause disruptions in water quality. Not only is there concern over the discoloration to the water, but this algae will also deplete oxygen levels at night. Unfortunately, just completing a water change is not enough to rid an aquarium of green water. Reducing nutrients (overfeeding and overstocking), turning off aquarium lights, and ensuring there is no direct sunlight will help prevent these algae from overpopulating. The addition of plants can help reduce nutrients like, nitrates, phosphates, and ammonia. If you have a bloom, the use of a micron or diatomic filter can strip the cells from the tank and utilizing UV filtration will kill these cells, reducing their population and green discoloration of your aquarium water.   




With an understanding of the components that affect water quality, you can take steps to prevent and address cloudiness and discoloration, allowing you to view the beauty of your aquatic life in crystal, clear water!

Reviewed by Petco’s Animal Care, Education and Compliance (ACE) Team

Petco’s ACE team is a passionate group of experienced pet care experts dedicated to supporting the overall health & wellness of pets. The ACE team works to develop animal care operations and standards across the organization and promote proper animal care and education for Pet Care Center partners and pet parents, while also ensuring regulatory compliance.