i doubt you killed them from lack of proper diet
coulda been any one of a thousand reasons
bliue cheeks and other sand sifters like the white, the diamond, often arrive at their countries of destination with a whole host of issues-from euronema to fukes, and internal parasites
often they come in, in excellent shape, ,for succesive shipments, and then for a month or so-they come in looking like living death, or look great for 24-48 hrs, and then start to 'pop'
they'll do fine on a pellet food-if you can get them to accept it, and no fish was killed by a diet of meaty frozen foods, even if of only one type, in that short a time frame-malnutrition in that sense takes awhile to manifest itself externally-and by the time it does, it's also usually easily fixed
all fish can be easily overfed-if nice tasty anything is around and 'gotten-able', it gets got
i believe it might just pay off to wait a few more months before getting another-this seems to be a time of year when many of the pathogens/parasites are more prevalent with incoming shipments from those regions-