what is the fish and what size tank is it in? if its not due to tank/water quality issues, then its just a deformity due to bad inbred genetics and shouldn't really affect its health much other than impairing its swimming a bit, but if its due to being kept in bad conditions then it is only the outward sign that even worse health problems are occurring inside from it being severely stunted and it will die an early death if not corrected immediately (and still is severe enough to have affected its overall lifespan, but quicker its corrected the smaller the effect will be)