Please make the pond as soon as you can. They can't be kept in a 50 gallon for long. It's better than the 20 gallon but it's still small. I agree with Animallove please consider moving the other fish out of the 50 gallon while the koi are in there.
There is a way to help you with the 50g with the kois if you can keep this tank under 50 degrees by adding ice their metabolism will slow so you will not have to feed them thus less waste.
