Hey, I feel your pain.
My mandarin just died. I have all the parameters that I needed, 100 gal tank, 100 lbs. LR, pods, etc. He lived for a couple days, then just died.
He was eating fine, didn't have any fish bothering him, etc. I looked at his body, had no obvious attacks, sickness, etc. Just died.
I know that my LFS didn't have enough food for him though. But the moment he dropped into my tank, he started eating like a mad man.
O well, I am chalking it up to a learning experience. A painful, sad one, but a learning experience just the same.