We’ve had some fish to die this year. One large goldfish, a large fantail shubunkin, a grey koi and a very large orange and grey koi, all females. The orange and grey one died just this week. We don’t know what killed any of them or how to find out.
The water quality of the pond must be tested immediately. It sounds like a nitrite and/or ammonia problem. If ammonia or nitrite is present, stop feeding the Koi immediately and add some Koi TLC to eliminate the poisons. Wait until the water is completely free of these substances, add an adequate koi pond filtration system, beneficial bacteria and start feeding very slowly to let the beneficial bacteria colony establish.