What causes cloudy water in aquarium?

if you have literally just set it up it is because of all of the dust coming off of the gravel, even if you thoroughly cleaned the gravel, there will still be plently of dust, but don't worry because it will settle within 48 hours as long as you are using a filtration system.

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Another reason may be an unbalance of chemicals such as chemicals used to fix the water and the chlorine in the water. To settle the white cloud you will need to leave the tank filtered and running for at least a weeks to two weeks, and to change the water after it has cleared and have it tested to make sure it clean of toxins.