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More than likely, the fish are knocking into something sharp enough to rub off a scale. Scales can and do regrow although slowly. Check your decorations as well as any other fish that may be in the tank that may be attacking these fish.
Keeping the water clean will help keep these "dings" from getting infected while they heal but if you see any redness or fuzziness at these spots, the fish will need to be medicated in a hospital tank.