Feeding your koi once per week or less is considered "underfeeding" your koi unless they have quite a good bit of access to green foliage, water plants, etc. You could suspend a light over your koi pond at night which will attract moths and other insects for the fish to eat, and feed less staple food or pellets. But if your koi do not have ample access to live and green forage, then the liability is great that you're underfeeding.