Jarrod, that really is upsetting - sorry for your loss. Hope the rest of your fish perk up.
I'm with you & Ash... probably a loss of BB between the frequent water changes/and then removal of fish... then again, 3-4 days doesn't seem enough time for a deadly spike, even if you set them up in a brand new tank. Not sure, just doesn't seem quite right.
Regardless, it's a mistake any of us could have made. I know I could have very easily have made it... and I almost did the other day when I was going to the other week. This doesn't make you a bad fishkeeper at all, infact you're one of the best keepers I know of!
Agreed with what Briztoon said, only so many hours in the day: in your busy position lately, I probably wouldn't have handled it nearly as well as you did... even if it looks like neglect now.
Don't want to repeat what's been said, so that'll be all for now. x