Guess or diagnosis ?
Let me say right from the start that it is just not posible to diagnose fish disease and health problems without first examining the affected fish and possibly the fish's environment. After all, would you trust a doctor who told you what was wrong with you without asking any questions or carrying out any examination?
would you phone up your vet and expect him to tell you over the telephone what was wrong with the family cat or dog ?
Obviously the answer to both these questions is no.
Trying to guess what is wrong, ( and that is all that people do if they don't carry out any investigation), is obviously the wrong way to go about things, unless of course you believe that aquarists and dealers have some form of mystical powers not possessed by vets and doctors! common sense and our own experience tell us that the right way to proceed is to establish what is wrong and why, before we carry out any treatments.