Its pH may now be too low due to the amount of ammonia produced from either overfeeding or waste.

Koi are also known to jump during spawning - so netting at this time of year is good.

Are they doing anything else ? Gulping for air or flashing against the side of the pond as this suggests a different set of problems.

If pH is low adding 0.2% salt will bring a quick cure without affecting the water temperature. A gradual change of filtered water is best tho