Depends if you don't mind losing the fish if you buy it and then treat. You must know what it is afflicted with and the med on hand . Buying and then figuring out usually never worked for me. If the fish isn't eating or shows sign of a thin belly area, I don't buy . They maybe to weak to transfer to your Qt system and further weaken if not eating . If very expensive , I don't buy . I have asked at some LFS to drop the price low enough that I don't mind trying to medicate. I just recently made an offer for a small tang for $50. The tang had skin flukes to the point of exhausting itself trying to scratch them off. They said $80 , and I rejected . So in about a week the fish will go to a point of poor survival. Even if treated, small tangs have poor survival rates when not eating or thin bellies. On the other hand if I purchase the fish before too long in the disease state and my diagnosis and treatment is correct , then lucky me.