Sounds like parasites? You can buy medicated food pellets if you do not have Prazi and a QT.
I personally like to Qt all fish with copper and 2 rounds of prazi before going into my tank. Believe me after loosing a battle to Ich 3 times I would not chance it again.
If I did it would be a fish from a local member,Or my LFS (Arc) they are very good,and honest as far as telling you if the fish is healthy,How long it's been their ect...
I think Kat is a great resource for you,If you do not want to take her up on her offer
I would at least be testing everyday to make sure the tank is not going through a cycle
with the tang in it.(that would be sad).. Ive never myself heard of a new tank completing it's cycle in this short of time but I know theirs lots of ways to shorten the cycle time.
If you don't mind me asking how did you cycle the tank?
Anyhow good luck and feel free to ask lots of questions! This Hobby is a learning process!
( I know Im still learning myself) and your in great company!