ok, here's my opinion,
although i'll probably catch crap for it....
i keep a very low level of copper in my tank (fish only)
at all times and i never have any disease problems...i did go through an ich problem that a Blue Hippo Tang caused , and lost about 300 $ worth of fish, so after trying everything that everyone suggested even on this message board, all the products failed, as well as hyp. treatment, it will work, but i had very little luck doing it, so i went with old school knowledge from a very good source...i started using copper in my tank, the brand name is Cupramine,,,you must test repeatedly and maintain the reccommended level for 14 days straight, keep in mind this is for fish only tanks and once you treat with copper, you will pretty much allways have it. again, this works well for us and it is my opinion , i currently have about 5 hermit crabs,naso tang,sohal tang,flame angel,lion fish,domino damsel,spotted hawkfish,niger trigger, and a clown fish, and they have all been doin very well with the copper, keep in mind, i bet about 90 % of all fish suppliers have a low level of copper in their sale tanks, ask them and if thier honest, chances are they'll say they do.