Did you add any new media packs of any kind. Like a Purigen pack or something? Sometimes even though rinsed, some will have super fine solids that go into the tank. Too fine for the filter to get out. IF it is an inorganic cloudiness, you can dose with API Accu-Clear. It's a flocculant that makes the fines clump together big enough for the filter to get. It's safe and you usually only have to do it once. Then, if the cloudiness does not go away, you know it's biological and can deal with it as such. OS.