Adding more fish to quarantine tank

[The Guy]

Originally Posted by clown lover

Hope the tang cops aren't around! but it's just not a good idea to put him in a 20 g long even on his own, I agree with Bill it will probably stress the tang out more. My 2 cents worth!!

Ok now we got the whole story, I probably would have done a rescue buy as well and try and help him get healthy. If a 20 long is what you have for him to get better in then do your best to keep your water quality up and I agree the garlic drops in the food is a great idea.
You may want to try some nori/seaweed on a clip, my tangs go crazy for it. But do not add any fish to the tank. Good luck hope the tang makes it.