how do you keep koi with red ear sliders , dont the sliders nip and bite them?
How big is your pond?
My koi are babies and they live with common goldfish, plecos, kissing gouramis, tin foil barbs,and even bettas, they are o.k. unless the water quality gets bad. there fins are the first to show it,, but this is in a crowded fishtank im sure in a large pond they would get along great with any same size fish, that can handle the temp of your pond...