im sorry for your loss

from what you describe, it sounds like it might have been a
pH shock or something to that effect...whenever we buy new fish, and although ph isnt usually an issue as long as its stable, its good to check the bagged water they came from to the ph levels of your tank to see if there is a huge difference...of course thats too late now but in the future, ive found that it helps a ton with proper acclimation and how long you should take to do so....
I think the food is perfect..omega one is a great fish flake
the other one could have the same effect but both also could have just been weak from the start...fish that school usually wont school with weaker buddies as its a setup usually for other predators....good luck with the rest of them!