I feed all through the summer but to be honest if the pond is established then they really dont need to be fed at all. The main reason we feed is for our enjoyment and to get the fish up where we can look for illnesses but it also promotes growth and there is really nothing like having koi eat out of your hand. As far as what your feeding them it all depends on how much you wanna spend. I feed an expensive wheat germ put out by hikari during the 50-60 degree phase but if I want to give them a treat, I throw in some unsweetened cheerios. Cheerios being wheat based are easy for the fish to digest just make sure that if they have small mouths that you break them up so they can eat it before it gets saturated and becomes waste.
~Yashin
P.S. sorry for the delay in the reply been working alot