Also, if your original fish that you started the tank with are outliving the newer additions, it may be they are immune to something they could have brought with them from the store and if the newer ones came from a different store where there was no disease exposure, this might be part of the problem.

Have you been quarantining your fish for at least 3 weeks before adding them? If you haven't, you should really think about investing in a 1-20 gallon bare-bottom tank. I would hate to think that this could have all been prevented...