Sick Fish??? Need advice

[ethuman]

so this is the first time ive encountered a problem with any of my fish and not exactly sure what to do.... Tank is a 72g bow with a 20g sump, been up and running for 6 months.
2 clowns
1 purple dotty
1 bangi cardinal
1 flame angel
1 sailfin tang
yesterday I notice the tang was moving a bit slower than normal and not going after food like he usually does. He has been in the tank for 4 months and have never had an issue before. this morning he is staying hidden in the rockwork and didn't even come out during feeding. I don't know what to do from here....
im in the process of setting up a qt tank for him now and think I should take him out??? is that right?

ammonia 0
nitrates 0
nitrites 0

any help would be appreciated

 

[e048]

Tank is too small for a sailfin tang. Did you qt any of the fish? When you say up and running for 6months has it been that way fully stocked or did you add something new?

I doubt nitrates are 0... Your test kit must not be able to detect trace amounts of it

 

[ethuman]

I realize the tank is too small for a tang. it was one of my first purchases before I took the time to slow down and read more. spoke to a lfs that would take him in trade when he got a little bigger. I have qt all livestock before going into dt and they have all been in dt for about 4 months together. looked healthy while in qt for 4 weeks. haven't added anything new other than a couple of corals that also went through qt