It could be disease, like ich, that starts in the gills. Two months seems like a long time for it to be an ammonia/cycling issue. When you get the clowns research how to do a freshwater dip, only do it for a minute or two in a clear container, if it is flukes (another disease), they will fall off and you will see them on the bottom of the container. Are you getting the clowns from the same fish store? Have you looked in all the tanks (water is almost always shared in all tanks) and seen if you can see disease like ich on the fish? For example, my local petco has ich in there tanks, I have seen new shipments come in fine and gone back a few days later to see them all covered in ich. Just some thoughts besides water issues, maybe it's a health issue. Hopefully you have a powerheads in the ank also?