I see you have the Hagen CO2 kit. Don't use the packages they give you, just use your own yeast and baking soda. It works much better. Also, put the tubing on the lowest possible notch on your CO2 ladder.
Also, your koi may seem to be doing fine, but you are woefully stunting them. If at all possible, put them in your pond right now, or as soon as possible. If you wait till they grow even larger, your fish will be stunted to the point of ugliness, and to the point where health problems may come into play.
You're in my heart, you're in my soul, you'll be my breath should I grow old