erfly illness (marine tank)?

55gal fish only(with live rock) tank. Established Dec/06. Water chem tests all negative including phosphate. PH 8.0-8.1. Temp 78. Fluval 403, protien skimmer, u/v ster. All other fish healthy. Noticed erfly (which is normally white with black stripes and some yellow) had her white areas turning grey/dark. Today she appears to be dying (labored breathing, at bottom of tank). I am quarantining her now but wonder if anyone knows what, if anything, I should treat for? (fyi: we feed marine pellet, mysis shrimp and live brine..pellet mornings, shrimp evenings). Again, water chemistry tests perfect.. help?