Is it just on the tail? And how large are the spots, like a grain of salt, or larger? If they are a little bigger and sit just at the edge of the tail fin, it could be lymphocystis. Ich will make the fish flash (scrape and flick against rocks) and may make the gills move quicker (difficult to get enough oxygen when your gills are covered with parasites). Lymphocystis doesn't do this. It is a virus, and generally doesn't affect the fish much if it is sparse. If it looks this way, just make sure the fish is well fed and as stress-free as possible, and it will go away on its own.