Overnight "white" cloudiness is a bacteria bloom. Less is more when it comes to additives to the water... I would not add anything to the tank other than, maybe, some prime.
Adding the sponge filter is a good idea not only because it will add lots of oxygenated surface area for "good" bacteria, but it will ensure your tank stays oxygenated when the bloom dies back and decomposes.
As has already been mentioned, take a day or two off of feeding and the cloudiness will go away. This is all a normal part of getting a tank cycled.
A UV filter is the nuclear option.