base on all the readings, here are my thoughts:
1. Bio pellets might have been contaminated by manufacturer, the often "bad batch" you may never know about before something happens.
2. New fish could be carrying some nasty parasite/bacteria with them, which they have grown immune to, but still a carrier of, and introduced to your old fish (which has no immunity toward said parasite/bacteria). On top of questionable bad bio pellets = further stress => quick death.
If i were you, i'd get rid of the bio pellets, or the bio reactor all together. Upgrade the skimmer, buy more chaeto with pods (but not from fishtown usa). Do smaller but more frequent water changes, like 10g weekly.
Hope this helps