Shy Koi fish

Question
I have Koi fish that only come out at night. During the day they are hiding. They only eat at night. How can I get them to come out during the day

Answer
It sounds like they are scared.  Are these new koi or ones you've had for a while that suddenly are scared?  How big is the pond?  How big are the koi?  What is the water temperature?

If these are new koi, be patient.  New koi are shy and scared at first.  Give them places to hide, and they will eventually be more bold.  It also helps to already have other fish in the pond (if it's not new) that aren't skittish.

If these are koi you've had for a while, and now suddenly, they are scared, then I suspect a predator has been visiting the pond.  The fish have thus become more wary and scared.  Since they are hiding during the day and coming out at night, that would indicate a diurnal (daytime) predator such as a great blue heron (if you're in the US).  If it were a nocturnal predator, like a raccoon, the fish would hide at night moreso than during the day.  Have you seen any signs of predators?

Good luck with your koi!