Most illnesses/diseases/deaths are due to stress, normally by other fish (overcrowding/aggression) or poor water conditions. If you keep good maintenance of your tanks, do not overcrowd your tanks or keep fish that are compatible with each other, you should have no problems. With that note, I had a slight occurrence of ich on 1 clown loach in my 55 gallon tank. I did not use any meds, but just raised the temp and added some salt (diluted in water and spread slowly over time). I also kept the lights on to about 2-3 hours per day, just for feeding. The reason is, ich is caused by stress. So if I kept the lights off, the fish would sleep, no stress. When we are sick, we normally rest to recover. The ich never spread to any other fish nor did I lose the loach or any other fish.
As far as WC, I now do about 40%-50% WC weekly in my 125 gallon tank. I used to do about 20%-30% in my 55 gallon tank.