It is difficult to tell the sex on younger Koi, it gets easier as they get older. Females tend to have rounder bodies and rounder pectoral fins and their fins tend to be somewhat smaller.
Males are sleeker, with more pointed pectoral fins, and their fins tend to be larger.
Others claim the colors of males are more brilliant. Older males have a sand paper like raspiness on the gill plates, and some claim you can also feel a roughness if you lick your tongue across the front of the pectoral fin. The easiest way is when there is a particularly aggressive male chasing the females, with the proverbial nose up the butt, you look for other males chasing the same now identified females. Inc. All Rights Reserved.