90-95% on all dieases, parasitic infections etc here. Key is in identifying the problem properly, get a GOOD book on the subject first of all. Then do not get caught up in thinking that you can only use meds marketed for fish to cure things. There are plenty of resources out there that will tell you how to safely treat a problem with another type of "med". For example, one of the most effective internal parasite treatments is as follows, 1g Panacur (horse dewormer) ground into powder and mixed into 1lb of beefheart or shrimp. Feed for 5 days or untill the fish stop eating which ever comes first. Then 1g Metro mixed into 1lb of food fed for one week. Wait two weeks, then another round of Panacur for 3 days. Water temp should be elevated up to the upper limits of the fish being treated as well, for discus I elevate the water temp up to 92 during the panacur treatment.
For most bacterial problems 1 cup of salt per 20gal of water, along with Maracyn or Maracyn II will take care of it.
Ich, same salt dosages, Raise water temp, and add quick cure at the dosage they reccomend.
First and foremost though, water quality is almost always the reason why fish get sick, weekly water changes should be considered a minimum. At the first sign of discomfort in a fish, do a water change. Sometimes you can nip things right in the butt then and there and avoid having to treat the tank.