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Are they boys or girls?
Female koi tend to have rounder bodies along with rounder, and somewhat smaller pectoral fins.
Males are sleeker, tend to be larger, and have more pointed pectoral fins.
Older males' gill plates have a sandpaperlike raspiness
Some claim that the males have more brilliant colours.
The easiest way to tell the difference is to watch their activity during spawning season,. It's generally the males that are chasing the females around.