I got my PBT a year ago this month. When I got him/her the tang was very skinny and had very pale colors. The fish was basicly malnurished. 1 year later the tang has at least doubled in size (probably closer to trippled) and has gotten very plump and its colors have become very vibrant and defined. The photo above is was taken a few months after I got the tang.
This is the most recent photo of the tang
Here you can see how plump and solid the body structure is
Continue to offer nori sheets. I feed 1/4th a 8X11 sheet daily and on weekends twice daily. I also suplment this weekly or twice a week with some meaty blender mush. Tangs are grazers and will pick at food even if its constantly offered.