This is the cherry barb from our fish tank that, until recently, we were calling Sinking Fish. He had something wrong with his swim bladder, and so could not stay buoyant except by expending effort. If you look closely you can see that his right pectoral fin is damaged, presumably due to being bashed against the gravel a few too many times. But now he is swimming around just fine; he seems fully recovered.
For the photo buffs: this is taken with my 50mm f/1.8 at f/2.8, with a +4 diopter on the front, at ISO 3200 to get a decent shutter speed (1/320th) to stop motion.