yep what Esther and Julles said......I give mine to local garden centers that have pond displays also, the local pond place I do business with trades me goods sometimes depending on the size, they use them as freebies for new ponders. If you don't want people coming to your house for a craigslist listing, meet them elsewhere with the fish.
as for the problem with the koi, if it is not predetor related, yes check the water chemistry....if you are on city water make sure you are using a dechlor. If you use chemicals in your pond or around it that could be a problem also, even an algacide or flocculant/clarifier, they can deplete the O2 in your water. Chemicals near the pond can leech in with heavy rains. check your KH also, if it is high that is good it will reduce your chances of major PH swings during heavy rains/snow/ice. PH swings can be fatal, as can high ammonia and nitrates and nitrites