Some fish may not adapt well to captivity. They are already stressed due to shipping and the hobbyist may not provide all of the nutrients through a varied diet the livestock requires. Getting a fish to eat prepared foods is one of the highest priorities for any fish keeper and this can be quite a difficult achievement. Research the fish you are wanting to purchase and have multiple forms of food available. Find the food your new fish reacts best to and try to feed multiple times per day, at least until the fish gets settled into its new home.