Well, the remaining cory is doing just great. The water levels are great... not quite sure what happened, but one fellow aquarist pointed out that it might be b/c my tap water and tank water are very different in pH. :( Very sad.
I had just finished cycling my tank with three serpae tetras, so we added a couple more serpaes and three really small Julii cory cats. They did extremely well for two days and I monitored the water closely.
Yesterday, I went home from work at lunch to find all the fish happily swimming around...