It's a very common mistake. I have done it myself. Carry your phone with you and look the fish up before buying if you can. Many LFS
are irresponsible when it comes to the fish they offer and the advice given. Most people are offended if you tell them they are not advanced enough to keep that beautiful fish alive. Even when it's true. Easier to just sell the fish. The good news is that you will learn and every year your tank is established, it will be easier to keep more difficult fish. But some are almost impossible even for advanced hobbiests. In my opinion, you haven't successfully raised the fish unless he got a majority of his natural life span in your care. It doesn't count to me that you kept a angelfish alive for a couple of years when they can live 20. But that's just me. I hate seeing beautiful fish being flushed, it's the part of the hobby the diver in me hates. So do your best and research first. It will reduce disappointments,which is what runs folks out of this hobby.
Butterfly fish, even copper banded, are not easy to feed. Some only eat sponges. I stay away from them because I don't have good luck with them and don't have the time to cater to their specific needs. Too many easier fish to raise.