Sometimes swimming against something that's a little abrasive can cause a slight injury. I keep Fancy Goldfish and I'll see a scale missing at times. I don't think this is serious. What's your tank maintenance routine? I change out at least half the water in my Goldfish tanks every week. I also add a teaspoon of standard aquarium salt to every five gallons of treated tap water. Salt is excellent for scaled fish. It boosts the immune system and helps heal injuries. I've used it for years in all my fish tanks. I also feed a little Spirulina every other day. It's high nutrient and helps with digestion.
"Fear not, my young apprentice. Just change the tank water."