Koi and goldfish both have a dorsal (top) fin, an assortment of bottom fins, and the caudal (tail) fin as well as a few others.

The dorsal fin in koi is extremely vulnerable to ulceration and it’s not uncommon for the mildest of bacterial infections to erode a hole right in the middle of it! Once the ulcer has healed, the fin can remain incomplete. When purchasing a new fish, inspect the dorsal fin to make sure it’s whole.

The two pairs of pelvic fins are the pectorals and the ventrals. These are the fins that define the landmarks for injection of sick fish because it is the safest site with good absorption.