I agree that not everyone knows everything about what they do. I don't know my entire job, or what I may have to do and I'm the foreman.
So if the OP talked to this employee and has a fairly new tank, trying his hardest to get corals growing and has a fair amount of frags in the tank. Just orders a new batch of CUC. Then one guy tells home to copper his tank to help his fish.
Sorry, but copper in the DT should be a basic no no.
In any retail type setting, the employee goes through a basic training before being let loose on consumers. do I expect them to be the end all on everything? No. But the basics are expected.