Im pretty sure that is a genetic mutation that can occur from inbreeding. Cant recall exactly but I remember finding information about it when I noticed it a lot in goldfish tanks.
"If a fish has a bent or curved spine, it is most likely infected with a Gram-positive mycobacteria (Mycobacterium marinum or M. fortuitum). This is commonly referred to as fish tuberculosis, piscine tuberculosis, acid-fast disease or granuloma disease. ... Remove any fish that appear affected."