My fear was validated when I took the castle out and out of it fell one of the mickey mouse platies... a corpse of one, that is. I'll see about replacing him on Monday. Platies are more of a hardy species than neon tetras, so it should be easier to do that without the problems that I had with the neons earlier in the year.
On the up side, I'm about to set up the tree. That'll pull me out of this mood.