First hex is completely normal in all fish. Their immune system will normally keep it in check. When you start seeing symptoms it is because something else is wrong. The hex is a secondary issue. The reason it seems to come back is that you are not taking care of the primary issue. Now for the bad news... I have no way of telling you what the primary issue is. I seldom have sick fish but if I do I send one to a fish pathologist and then treat accordingly.
-john