The nitrogen cycle makes keeping fish in your aquarium possible. You can read up on it by following the cycling links in my signature.
What are your water parameters and what color is the cloudiness? If you're unfamiliar with water parameters, get a liquid reagent test kit like the API Freshwater Master kit and measure your ammonia, nitrite, and nitrate levels. I have a feeling that your tank is going through a mini-cycle and the cloudiness you're seeing is a bacteria bloom.