Sounds to me like he's stressed out (makes them pink)even if he's sick , it's no fun in solitary confinement. Give him a gold fish for company, put more water in, and cover the tank up, he wont panik so much in the dark, and cant get out if he's safely covered.Dont dose the water with any thing more, that's the quickest way to kill your fish! Call your local vet and ask him who is the closest koi vet in the area, then ring there for professional advice, what to do next.
Good luck.

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