i dont think iw ould suggest taking a fish to the vet. how many vets do you know that will see fish?
but also, it can be very stressful to catch the fish, and then transport it in a small container.
wehat i would suggest it post a picture on different boards like these and a list of symptoms, as well as your water levels, maitinence, other livestock, previous history of disease of infection, ect, and someone will be able to ID the problem, as well as suggest a medicine or remedie.