Alright.... it's morning now, and neither fish made it through the night.
Gonna try for attempt #2.
Several people expressed concern about my tank cycling. I don't think it's what killed them, but I'll make sure to do it right this time.
Going to follow these instructions:
http://www.monsterfishkeepers.com/forums/showthread.php?207575-Fishless-Cycling
I'm trying to think of what else I might be lacking. Does the HOB filter oxygenate the water enough? Or do I need to get one of those bubble things?
That's all I can think of for now...