It does seem to me that it is related to the condition of your water. Even if it isn't the water, water changes help out tremendously in healing fish injuries.
How long has the tank been set up?
What are you feeding and how often?
How long have these fish been together? Were there any new additions?
In a 200g tank, it would require a lot of medicine to medicate the whole tank. You might want to pull fish out and place them in a hospital tank for bacterial treatment. There are meds out there that hit both gram negative and gram positive bacteria.
Moreover, salt is usually very helpful for fish as well as the use of melafix.
Good luck!