my powder blue tang had for 6 month 5 inch start to look skiny he is been eating fine all kind off food may be i start to feed once or twice for last one month due to phosphate over .6.
u shouldnt be feedin once a month. u need to feed every day but u can skip 2days. right now u got that fish on a liquid diet. feed it with garlic. u need to keep ur phosphates in check.
I use half a sheet on a magnet attached by a rubberband. Twice weekly
Mixed frozen foods twice weekly
Dry foods (pellets soaked in RO) twice weekly
Also ck if other fish are hogging all the food. You may want to feed in intervals. ie Feed, then wait a few mins then feed again, then wait a few more mins then feed again etc.
As far as phosphates, find another reason or better phos reduction. I'm partial to bio-pellets.
good luck
I feed them twice and 1/4 sheet of nori i will increase it to 1/2 sheet i feed them daily mixed of massyes and plankton and brian shrimp with vitamins and pallette and some time flakes