Also tell us more about your tank:
How long has it been established?
What size is it?
What fish (and numbers) do you have in it?
What are your water change practices (frequency and amount)?
What kind of filter are you running, and have you done any maintenance to it recently?
Are you using a chlorine/chloramine treatment with your tap water before adding it to the tank?
Have you done anything different recently?
The cloudiness may be a bacteria bloom. This might occur if you've accidentally wiped out your nitrifying bacteria, and they're trying to reestablish themselves. If the bacteria has been killed off, the ammonia and nitrite levels in your tank may have becom toxic.