to answer your question on scales... it is not normal. post in the sick forum to see if you ought to add some FRESH WATER /aquarium/ salt or not to prevent sickness P:
missing scales are like ripped fins, they can be scraped off by a plastic plant, or drop from stress or bad water. if your tank stuff is fine he probably just scraped it off. watch him close though ^-^ also if you notice more missing, strange spots, problems with his fins, or odd behavior, post in health immediately to get help because he's probably sick D:
check the tank to see if there's anything he may have knocked it on, and if he has tank mates ( most don't and probably really shouldn't XD; ) make sure they're not picking fights.