You might want to look up symptoms of disease--some fish will do that when they have a parasite or fungus. If they also have white spots, it might be Ich.
Somebody posted this web address: http/www.fishpalace.org/Disease.html on the emergency fish forum--it has symptoms & pictures of fish diseases. Turns out that scraping is a symptom of a bunch of diseases.
I wouldn't add any medication without knowing what causes this. Get a microscope, strong magnifying glass, or photographic lens, and see if there is something strange floating in the water. My fishies were scratching too, caused by tiny little micro-orgs that I now filtered out. http/fish.orbust.net/forums/index.php?showtopic=9024
The commonest reason for fish to start flicking and scratching in an established tank is water trouble. Check that before thinking disease. Fish are pretty robust, as long as their environment is okay.