With good water quality and high mineral content water, health problems for mollies are almost non-existent. Regular water care is a great disease prevention approach for most of the fish that we see in the hobby. In the past 7 years, since I got back into the hobby, I have had one outbreak of ich in some newly purchased bettas and no other health issues. I do have fish die of old age from time to time, but illness is almost non-existent in my tanks.
As far as a diagnosis for a specific illness, CuriousK, I would post a good description of symptoms, and a picture if you have one, in the Tropical Fish Emergencies section. People in that part of the forum help others every day with illness and are far better at it than I would be. Having read a few posts there, I find that they often are advocating clean water as a cure for many diseases.