Welcome to the forum from Plymouth UK Fedfire 37, koi can continue to grow all of their lives and will out grow your ponds or be stunted by them .
You may want to concider
@MitchM suggestion of swopping them for goldfish as in reality you need a pond of 1,000 gallons upwards .
But in saying that I kept koi indoors behind glass in a huge 220 imperial gallon tank 6 x 2 x 2.5 for 22 years and my koi where in imaculate condition , however we moved outside 6 years ago to a larger pond giving the 3 or 4 times that space .....see photos of 220 gallons imperial set up indoors
Ignore the 50 banners that was eight years ago now "god how time flies"
As the koi reached 22" we sold them on but kept two fish back Tiny a rescue koi and browntop as we call him , Tiny passed last year aged 27+ [the plus ign is because you dont know how long they had kived with the breeders prior to selling to koi dealerships in the UK]:cool
The large koi with the douitsu scales is our oldest koi at 28 years + this year, we still have others that are 15, 10, and younger from those days they never grew over 10 ".
He has just survived a lightening strike to the pond that has left him a tad kinked but otherwise in good health .
Dave