Wow, sounds like what mine was doing a few days before. He always swam "excitedly" whenever I walked into the room, and he seemed to always have trouble with the current, but he was a tiny false perc. His belly got bloated, the middle and backside of him looked thin. He ate fine, and I would notice that between feedings, he would hover over my live rock and pick at things that he saw. Sometimes it looked like he would eat something off of it, sometimes he would spit it out. I thought that all of this was normal and maybe it is, but this morning, I found him on the bottom of my tank. My hermit crab had just gotten to him after he died. When I removed him, there was not a mark on him, no spots, or anything. I don't know what killed a "seemingly happy and healthy" fish. My parameters were all fine and other tank inhabitants are fine. His belly still looked bloated though, so maybe he ate something "bad" off of one of the rocks? I don't know....
Good luck with your little one and do let us know if anything happens.
