If I'm reading your posts corectly, you have at least 1 koi that is 12" long, and other fish in 6.6 gallons of water? I don't know that you'll ever have success if this is true. A larger water environment is essential or reduce the fish count to one small fish. Even that seems cruel. Koi grow large, gf can be but are a better bet for an aquarium, but not at the size you have.
You never answered exactly Meyer's questions, which would clarify what a possible solution might be.
btw, without knowing the water quality being used, IF it's just chlorine, it can be dissipated by air exposure, 24 hours or so. If it contains Chloramines, the airing out process won't help you as it sometimes takes up to a month and then, there's still remnants. If the latter, you should use the anti-chlor.
If you're finding you have to do drastic water changes to keep the fish alive, you're doing something wrong, be it overcrowding or over feeding or not enough water movement or not enough oxygen in the water, or...you see the point. You shouldn't have to do this if the system is in balance. Find the imbalance and correct it.
Just mho.