I wouldnt worry about it. There's no way to push the Co2 level up high
enough to hurt the fish without Co2 injection. Co2 injection is used
by aquarium people for planted tanks. It would def help out the
submerged plants in koi ponds but you'd need one heck of a co2 system
and you'd want to keep water movement down to a minimum. Most people
move the water around a lot in koi ponds to get more o2 in the water
because they generally overstock fish.

Water will contain the same amount of co2 as the air. Any water
movement will bubble off any "extra" co2 that's added to a system
(just like shaking a can of pop).

Sam

Oh that's good to know. Thanks. Hmmm. So why did I buy that CO2
kit???

Joan