Sorry for your loss though remember, You have done nothing wrong this does happen there are specifics you want to look for when purchasing fish One is the tank clean, is there compatibility, will the shop owner let you watch them eat, are they swimming, do they look sick or diseased, and when you do get them you want to have 2 tanks up one for your show tank and one for a quarentine tank set them in the quarentine tank for two weeks observe them is they do well go ahead and put them in your show tank if they do not there is something wrong with them may it be disease, or parasite. Hope this helps.