If your fish tank has gone cloudy, don’t despair, it’s quite common and is unlikely to be anything that is harming the fish. Fish don’t seem to mind; in fact it’s the tank owner who gets more upset simply because all their efforts are disappearing into a white fog.
I’m going to assume that your tank has been set up for a while and rule out the cloudy fish tank water that can happen if you don’t rinse the substrate thoroughly before setting up.
That really leaves a couple of options depending on the color of your cloud. If it’s white it is most likely caused by a bacterial bloom.   This is a sudden increase in bacteria …