Hi tone
If You want small Koi to grow these are the important factors You need.
Heat to 26 to 28 degrees this is the best temperatures to get a koi matabalism running perfect.
Loads of air, this will aid in water quality and increase growth by atleast 30%
Good filtration
Good quality food
Regular water checks
And the one thing that I have found to increase My Koi growth is current. This keeps the koi on the move all the time, it increases there matabalism rate, so they are always healthy and very hungry.
This is where that good filtration comes into play.
I have had some terrific results useing the above. Last year I grew a 5 inch fish to 18 inch in twelve months

Justin