Very sorry about your loss
1 how long did you have him?
2 how long did he live in that vase?
3 Pellets are a fine meal actually, it is always best to substitute some sort of other food with them a few times a week, like bloodworms, brine shrimp, even regular shrimp cooked then chopped up very fine.
I had two boys who sorta went like yours. Even when you do everything right something can go wrong that we don't know about. Could be he was sick before you got him and didn't know it. I had two boys 1 died in 3 days of having him, the other in 4 days of getting him. Both were from the same LFS and same shipment so to me that tells me there was something wrong with the shipment and they had some disease before I got them.
All 6 of my boys are healthy and happy now and 1 of them is from the same LFS