Cloudy aquarium water may be the result of microbial bloom, which can be caused by adding too many fishes at once. Water may also turn cloudy white or gray when water is initially poured over sand or gravel. Green cloudy water occurs when algae blooms as a result of excessive nutrients in the water. Yellow cloudy water is the result of driftwood, fish waste or dissolved organic carbons.
Sand or gravel dust turn the water murky, but the dust settles after a few days. If the water remains cloudy white or gray, wait for the problem to correct itself. The water clears up when enough beneficial bacteria grow in the water and in the filter media. This process is known as the nitrogen cycle. It takes between two weeks and two months to complete, ensuring the survival of fish in the aquarium.
Light promotes the growth of algae. If the water is cloudy green, disable the tank light until the water clears up. Also, change out 25 percent of the water every day, reduce the amount of food given to the fishes, and change the filter media. If the problem persists, test the tap water for nitrates and phosphates, and add live plants to the aquarium.
To clear yellow cloudy water, regularly perform partial water changes, and use activated carbon to remove dissolved organic carbons and other impurities. Change the activated carbon every three or four weeks.