What happened to my angelfish?

I just recently lost one of my angelfish due to something I can't pinpoint. He was one of 3 juveniles I got from an angelfish breeder. They were all doing well growing up in my 46 bowfront. I had them for a month or so now while maintaining my usual perfect water parameters like every other plant tank I own. I feed the New Life Spectrum .5mm pellets for the most part, Occasionally some ken's flake or frozen bloodworms. Well my one koi angel stopped eating 3 days ago. He was acting normal still but just wasn't eating any more. This went on for 2 days until he not only stopped eating, but now had no real equilibrium and couldn't swim correctly if at all at times, but was still breathing and alive. As of last night, he died. What could of happened? Nothing in the tank has changed and he didn't have anything weird looking about him that I would of noticed. Just the not eating and swimming problems seemed to be the only symptoms before passing. I'm worried that maybe it was constipation, bloat, or swim bladder problems. Even though New Life claims to say the pellets are safe and could be fed exclusively because they are balanced, could they have been the problem. Could he have been over fed? The angels liked to eat at the surface, like when the pellets would float. Was this bad? I'm trying to stop them from doing this since now I'm worried. Could the pellets constipate him even though New Life says it can be an exclusive diet? The only thing I did once I came home from work to seem him swimming all crazy and getting blown around by the current was I gave him and epsom salt bath to see if it would help but I think it was too late and too far progressed. The bath didn't seem to help although in my opinion, he did some of his best swimming when in the bath. What do you think went wrong? I'm so frustrated and upset about losing one of my angels. I give all my fish exceptional care so this is bothering me now.