Have you tested for ammonia and nitrites? It kind of sounds like the tank is not fully cycled. If you have ANY presence of Ammonia and nitrites then the tank is not fully cycled and that could be causing the problems.
Also, Quick Cure is a harsh med and mainly used for ich. Do they have white grainy spots on them?
I don't know too much about koi so I'm hesitant to give too much advice. But I do suggest you test for ammonia and nitrites and also a do a large water change (30-50%). Then get back with us with test results. Sorry I couldn't be much help. You should get some more replies.