I wouldn't use any chemicals in the QT at this point. I'm a big fan of hyposalinity and currently have two tanks running hypo for some new additions to the DT. But for your case, I would say don't do anything yet (no hypo or chems) since your clown is eating. I would try to build up their immune system (feedings, water quality) before trying hypo or chems and just observe feeding/swimming behaviour. But remember, just because you don't see the ich doesn't mean it's gone. It's just fallen off the host to reproduce and will be back soon. If I see ich on a fish, I try to work on their immune system and once the ich is gone, that's when I do Hypo since that treatment (and any chem treatment for that matter) is only effective during the free-swimming stage of ich (not on the fish's body). Good Luck!