Blonde Naso not eating

[newsalt]

I have a 4-5 in Blonde Naso which I've had for about 3 weeks. Suddenly over the past 3 days it hasn't been eating. Before, it would love Nori and flake, but now it doesn't go near the Nori and the flake food floats right past him with no reaction. It's getting pretty thin and I'm getting concerned. I bought a small bottle of PraziPro, but haven't used because of all the mixed stories I've read about it. My tank is basically FOWLR, but has a RBTA and one or two LPS. Any idea what I can try to save the fish?

 

[elegance coral]

Have you dosed any chemicals to the tank?

 

[muskiewade]

Do you have a quarantine tank to move it to?

 

[newsalt]

I don't have a tank to move it to. I just dosed 1.5 oz of PraziPro. The tank is 180g.