Reasons you have sick koi
The most important thing you need to know is that when your koi are sick, you must ask yourself, "What Did I Do To Deserve This?"
And usually the answer will be in:
And, once the horse is out of the barn, you can shut the barn doors all you want, but you must act, and act definitively.
Perhaps the best way to do that would be to use Aqua Meds FREE "HELP FORM"which supplies step by step instructions and information on how to recover the fish you're working with.
If your losses are radically fast, then you're either dealing with water quality issues as suggested above, or perhaps something of a Koi virus, like KHV.