Update:
Well, I still have the yellow tang and the two purple firefish in QT, and I have come to the conclusion that the symptoms I saw in the yellow tang, that I thought were representative of septicemia, were probably due to ammonia poisoning. I think I compounded the ammonia problem by dosing cephalexin, which further inhibited the bio-filter. I am starting to think that the original source of my ammonia problem was the CP, which somehow impaired my bio-filter. Has anyone ever had CP adversely affect their bio-filter?
As for the two clowns my DT, I started feeding pellets that I soaked in CP, dried, and then soaked in Selcon. They eat them very well and I have not seen any signs of them flashing or scratching in about a week. I am going to continue feeding the CP food for another 4-6 weeks. I think that should be enough time to eradicate any ich that is still in the DT. Also, I think I read somewhere that CP has a relatively long half-life in fish and should offer residual protection for the fish after treatment ends.
Anyway, I am hesitant to try CP again after the way the yellow tang responded to the first treatment. He is doing much better now and is just starting to eat again. I need some encouragement to give CP another shot. My only hypothesis is that the QT had a lot of algae in it and when it started to die because of the CP it caused an ammonia spike. I am hoping now that because there's no algae left in the QT that there won't be another ammonia spike. What do you guys think?