I didn't realize the feeder was alive in there ..... he's much smaller than the fish that died. makes me wonder if lack of oxygen is what killed them. do you have an airstone in there? I know that certain meds and even conditioners will decrease oxygen levels in the tank. could this possibly be the case? what dechlorinator do you use?
Sorry for your loss. I would be suspecting lack of oxygen. I would also check for a faulty heater. Heat with insufficient surface agitation would easily kill of the larger fish first.
I once had a tank of mine up and running and all the fish, pbass and hampala barbs all started gasping for air, so I did a huge water change and saved them. my guess is lack of oxygen