Medicating the tank is obviously the last resort. The best way of preventing illness, especially parasites is regular tank maintenance, buying healthy fish and if possible, putting them into quarantine before adding them to a main tank. Not many people have got quarantine tanks, I haven't, so just choose your fish very carefully and only buy them from reputable fish stores. I hate fish shops that have dirty tanks,if you're going to buy an ill or diseased fish, it's going to be from one of these places. If they can't be bothered to maintain their business properly, they won't get my business, it's as simple as that.