Hi,
Was your tank recently set up (with in the past few weeks?)
What color is the cloud? White/grey/green?
If so then it is likely a bacterial bloom. Aquarium filters need to cycle and this can take 6-12 weeks. Cycling a filter means you are building a beneficial bacteria colony in the filter - it is this bacteria that breaks down the ammonia (their waste) so it is less harmful between water changes.

Right now you are going to want to do daily partial waterchanges and make sure to condition the water with a dechlorinator to make the water safer for them.

How large of a tank do you have and what type of filter?