I am about to give up

[badasstang]

So, I thought I have finally figured this hobby out, for the most part atleast. Well I bought a blue tang about a month ago. Most of the previous fish I have had die, died from Ich. Well this blue hippo was doing great, eating awesome (prime reef and formula one). He was always aggressive towards the damsel, so I thought that was a good sign. I fed them last night, he ate well...woke up this morning and he was on his side on the bottom of the tank. I gave hima freshwater dip (told to do this if they are looking really bad), and put him in the QT tank, went to the store really quick, came back....goner.
What could make a fish die in a night? He didn't appear to have ich when I fed him, or throughout his short life.

 

[badasstang]

The damsel has been in there for a few months now.
If the tang got ich, could it be non visible, and make him die just over night? Like I said last night he was doing great. I did forget to say something, he was really skinny, like you could see his ribs, but he was eating good, so I wasn't worried about it, just figured he needed to pack on some pounds. When I did the freshwater dip he was already on his side, I actually picked him up with my hands because the net could get to the area he was at, he didn't try to swim. When I put him in the QT (which the salinity was lower by a couple points), he was trying to swim, but when he moved his fins, he would go bottom up....almost seemed like only one fin was working good. I have no idea.
If I move the scooter blenny, and damsel to the qt for 30 days to get rid of ich...will the rocks, corals, inverts, or sand carry it as well? Also, there is a starfish in there, what about him?
If I put the two remaining fish into the qt, should I do the hypo on them? What will the scooter eat, doesn't he eat pods from the sand?
Sorry for all the questions, just trying to figure it out!