Both of these fish need veggie matter on a daily basis, especially the tang. One thing I prefer especially while in a QT is frozen broccoli fluorets. They remain intact much longer and are far less polluting than nori sheets. Don not use fresh veggies though, they must either be blanched or prefrozen to soften the fibers. Fish cannot digest raw veggies. You can simpley use an algae clip and let it stay in all day. It will take the fish a little while to get accustom to it but they will eventually. Just be sure you start with a small amount and change it daily, eaten or not.
Once the fish are back in the main display you can continue this or choose something else but they should not be on a mainly meat diet. Both of these fish will be much healthier with a stronger veggie diet. Any other fish you have will benefit from veggies as well.