I would suggest a few water changes to get those nitrates down. 20-40 ppm
is a little broad ranged but non the less high. Be sure to use well aged/areated SW
and do a few 10-15% changes over the next week or so.
As far as what killed the Banggai, it's only guess work. The details provided so far do not point to any conclusions. Since you do not appear to be using a QT
for new fish, be sure to keep a close eye on this one for possible problems.
Do you know if these were both males, male/female or both females. Could be one killed the other, any sign of aggression between them previously?