My advice would be to learn as much about the fish you are interested in buying before you buy. Are they top, middle or bottom swimmers, are they shoalers, are they flicking, have faded colouration or not behaving properly. These are some of the signs that importers look for in order to try and diagnose problems and get a good guess at a treatment. There are many other signs as well but if they are not "acting normally" don't buy.