Here is a video of a yellow tang doing what appears to be voluntary back flips.  The specimen appears fat, disease-free, and not under any distress.  The owner report his tang performs its circus act randomly and irregularly (says the owner: “He just gets in his silly moods sometimes and does this.”)

https://www.youtube.com/watch?v=knEBiSyP7ko

There are many more anecdotal accounts of yellow tangs performing similar “tricks”; just do a search for “tang flips” in any saltwater aquarium forum.  A year ago, Reefbuilders reported about a yellow tang that seems to enjoy swimming upside down (video below).

What is it about this species that makes them perform these strange acrobatic feats?  Is this just yellow tangs having fun?  Showing off?  Fighting their reflections?  Buoyancy problems?  There are not enough reports (both in captivity and in the wild) to say these acts are natural behaviors for yellow tangs.  Yet, there are too many reports to think these acts are isolated phenomenons.

https://www.youtube.com/watch?v=WgA1BpA8RaE&x-yt-ts=1421914688&x-yt-cl=84503534&feature=player_embedded