They are a fish with many names, black skirt, white skirt, black widow, widow tetra - and glofish tetra if they have had jellyfish genes spliced into them. Gymnocorymbus ternetzi is the scientific species name. And if you check them out, they grow rather large for a tetra. That one just has a head start.
I think it has an internal parasite or possible swim bladder issue. If it swims fine and upright than I would bet on parasite. I don't know what you do to treat that..usually its fatal that advanced.