Is the cloudy water in the tank itself or is it the tap water thats cloudy?
But in this case am going to assume it is the tank water thats cloudy.
There could be a number of reasons for this, but the most common reason of cloudy water is a bacterial bloom.
Bacterial blooms occurs when the tank is forming new bacterias, this commonly happens in new tank set ups when just starting a fishless cycle for example.
Can you provide a bit more information regarding your tank, such as is it cycled, what sized tank, how long has the tank been running, what filtration you have and have you replaced or did a big clean of the filter and/or media, any changes to your tank perhaps the addition of a number of new stock?
The more information you can provide, the better the idea of your set up we have and the more likely to be able to help you.