I wish I could agree with this. Frozen fish food lasts a lot longer than 90 days. I would place food poisoning low on the list. There still are many different reasons why the fishes died. Do you put on any lotions on your hands, even a trace of soap can nuke a tank. But I think I remember you use gloves, but always? I looked at your pictures again and the fishes slime coat looked pretty ragged. Of course, there are many causes for this, but one is shock. Temp swing or big ph swing could cause. Also too rapid acclimation when introduced. And you could even have bought fish that weren't treated well during capture or shipping.
Try a different LFS, have them double check your water and buy a really easy test fish. Acclimate over at least an hour or two, see what happens.