Skinny fish-internal parasite?

[Kahuna Tuna]

I have a doliatus rabbitfish that I have been struggling with for about a year now. He was once nice and plump but has been gaining and losing weight and seems to be trying to fight off what I believe to be an internal parasite. He was on deaths door about 9 months ago and had wasted away in a few months to the point I figured he was gone. He has constantly fed well and eats everything and a round of Blue Lagoon medicated food seemed to bring him around and he began to put weight back on. Lately he has been losing weight again and does not seem to be responding to the meds like he did last time.

I have tried medicated foods with prazi and metro and while these seem to help some and have kept him alive to this point I wonder what I can be missing here to help him.