Assuming "mega red" is some color variant of A. cacatuoides they should be relatively hardy and easy to keep.
Hard to know in hindsight but I would expect the common causes for death shortly after acclimation to be:
- The fish already had a disease or internal parasite
- The tank has going through a cycle or mini-cycle
- There was some contaminant in the water
- Poor treatment at the LFS or in shipping to the LFS
- Poor quality, hormone fed stock
- It hadn't been eating for a long-time(should be fairly obvious from looking at the fish though)
- Aggression from some other fish in the tank
Basically, all the same things you could consider when any fish dies.