Well don't let them say that, and invest in some liquid test kits of your own, as bad water quality can make them flick, have you just preformed a water change, right parasites, any grains of salt on the fish, red inflamed gills, or laboured breathing, plus a yellow to gold dusting on them, you say that one of the fish has whitness of it's body, well can you describe it is it fluffy, any grey mucas on the gills, whitness on the body can mean parasite or bacterial, but your tank sounds like it has alot of problems, there is alot of sick fish in there, how often do you maintain the tank.