How often and how much do you change when you do a water change? Where did you get the tank from? If you can QT them for weeks and then they die in a short period of time I'm thinking the tank or acclimation process, not the source of the fish. Maybe switch the heater from your tank with what you have in your QT and same with the power head to eliminate those things. You could pickup a grounding wire for a few bucks and eliminate that as a cause, plus it is good to have in case it develops in the future. Regarding the tank, maybe there is something leeching into the water from the tank, can't think of what. Might be a pain in the butt, but maybe you can switch out the tank. Can you post a picture of the tank?
Also, maybe change out the food you are feeding them?