There is a product by Seachem called Purigen. I know of people who used this in their filters (it is really tiny stuff, so you need a bag to put it in that is very fine mesh, finer almost than pantyhose, I think you can get some that work from Seachem) and then their water would be crystal clear. I only recommended this product when no harmful cause for the cloudiness could be found, but people who used it said it was wonderful, and it really cleared up their tank. I'm not really sure what the product is or why it works, but I know of customers who really really loved it. If your fish are all healthy, the water all test well, and the tank seems fine, you might try this.
Emily