koi eat koi normally when one is dead, then other get into the meal. Really do not know who cut the flesh out. the red part is ulcer start to surface up. If you really want to save the fish, q tip iodine and spread over the cut, cover neoprorine then cover with dental powder seal. realease the fish to the pond then two days latter check again, if dental powder broke off, wet the wound then pray dental powder again. (dental powder can only work when it contact with a wet surface). Since the wound is large, chance it will make it is slim.