I understand, this is why I think the key is prevention, which the medicated mineral block I noted can help with too.I'm not a fish expert, many times it's difficult for me to diagnose what the disease is.
I try to be careful and buy from sellers with good feedback but that doesn't always help. I lost almost an entire batch of expensive rainbow fish by following the advice of not treating unless I can diagnose EXACTLY what's wrong. Looking dull colored and not having an appetite really doesn't help me narrow it down.
As many steps as you can take to prevent a disease/infestation is very important.
Buying from sellers with good feedback/reputation is certainly a good start
Not to keep pushing information from this same website, but they have an excellent article that provides a good outline for aquarium fish disease prevention
Please give this a read:
http://www.americanaquariumproducts.com/Aquarium_Disease.html