Goldfish with missing scales?

I have several goldfish in my 55 gallon tank varying in age and size as well as type. They are all eating fine and seem healthy. It has been set up for about a month. Recently I have noticed that the majority of them have scales missing and I am unsure if this is cause for concern. I recently added an 11.5 inch... show more I have several goldfish in my 55 gallon tank varying in age and size as well as type. They are all eating fine and seem healthy. It has been set up for about a month. Recently I have noticed that the majority of them have scales missing and I am unsure if this is cause for concern. I recently added an 11.5 inch plecostomus to the tank and have heard that this fish sometimes can prey on goldfish if it is hungry. I feed him algae bites and also cucumber and lettuce and he doesn't feed often from what I can see. I do not think that he is going after the fish and don't know what else could be causing the goldfish to be missing scales. Does anyone know what might be wrong with them?
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