How big is your tank? When you said you get your water tested does this mean the shop are checking it for you? If so the first thing I would do is buy a liquid test kit and test your water yourself. Get an API liquid test kit. Scratching could be anything at this point and disease are almost always related to poor water quality. I would not trust your local fish shop to do these tests accurately. You need to do it yourself. When you know the results we can start focusing on the possibility of parasites or bacteria. It might just be very high nitrates irritating the gills.
High nitrates, raising temperature and adding incorrect or unnecessary medications will stress the fish. This will make their immune system weak and they will then be susceptible to all cants of disease.
We must eliminate the water parameters ammonia nitrite and nitrate first and we need to know the ph of the water.
