What kind of water do you use? Tap water? Then does your water come from a well or ?????? Some water has tons of minerals in it and can make water clouded at times. this could be the cause. What are you using for a water dish? This could also be the problem. Plastics can hold small micros of cleaners/soaps. Not that yours is or it could be that the combo of the "hard water" and your dish just doesn't work out.
Then like it has been stated before what is in your tank? do some tests i would with your bedding and other things in the water and that dish and other poss dishes you might use.
I don't have this problem but I know we all have diffrent water in where we live. Not that this could not happen to me.. It should not be to hard to figure it out with a process of elimination.
Good luck