IMO the best way to treat it is to make sure the fish is in a stress free environment. If that can't be done then transfer it to a seperate hospital tank for a hyposalinity treatment.
Well there is no signs yet, but my blue hippo scratched it's side on the lr. Only once, and thats all ive noticed. I just got him today and after the acclimation he was ok, and than maybe 3 hours later he did it. The hippo looked good and ok at the lfs. Is this maybe normail behaviour for blue hippos? If it turns out to be Ick do i need to treat all my fish in the tank?
Ich is highly contagious and if one fish has ich then it would be prudent to assume that all fish have ich.
If you want to gaurantee an ich free tank than you should set up a quarantine tank and treat with either hyposalinity or copper ... leaving the show tank without fish for 5-6 weeks will insure that any ich within the show tank will die off (ich need a fish to continue their life cycle).
Heres a good article on ich ... within the article are separate links under the treatment section which will discuss how to use hyposalinity or copper (cupramine) .. there is also a section discussing how to put together an inexpensive but effective quarantine tank.
Blue tangs will also develop spots and scratching if there's any parameters out of wack with the water. First test the water to make sure all the params are good, also mske sure temp is acceptable. My blue tang is the first sign of anything at all wrong with the water