Yeah, short bodies aren't really "short" per se. Maybe you could put your fish into a new, larger tank? Maybe he doesn't like the conditions of the 55-gallon? Check water parameters. Are your other fish acting strange? That is another sign that your tank parameter/water quality isn't ideal. Also, if you feed him daily, maybe hold off for a few days (2-3) and see if he is just bloated from food. Check for any signs of disease. Is he acting alright? Is he swimming oddly?
It could be something simple like genetics; consider where you got him. Did you get him from a quality fish store with quality fish (quality fish store meaning healthy tanks suitable for the stock, healthy specimens who are active like they're supposed to be, very few to no dead fish in tanks, etc.)? Or did the store look suspicious with semi-healthy specimens (semi-healthy means fish were in semi-ideal conditions, and many fish were dead in various tanks even with healthy specimens, etc.)?
Also, you can get your water tested by any local fish store. Maybe you have a mom-and-pop fish store you could go to? Or you could even go to a "normal" pet store with fish for sale.
I hope this helps! Good luck!