I stopped by PetSmart today and asked about their water params and if they've been having any issues with their new stock of Serpaes. Params match mine and they haven't seen/heard any issues. I talked to the girl for a few and we stumbled onto something... the plants.

I would suggest that my tank is heavily planted with cabomba, anubias, swordplants (big & small), and ludwigia. She mentioned that it could be oxygen deficiency. I didn't think of this, because of my filter & pump, but it's possible that there's just too many plants which are giving off CO2 at night, and the air flow doesn't compensate - which could be why I only have a couple of deaths every other day. It would make sense that that's why my fish are only dying at night, as well.

Soo... I purchased a new air air pump with a couple of air stones. I'm hoping these will help alleviate the O2 problem and save my fishes!