What do koi fish eat?

They eat fish food.

You can choose to not feed them at all because Koi are bottom feeders, and the more you feed them the less algae they will eat. Feeding koi is done only for entertainment reasons. Though if you do feed them, we recommend not feeding fish more than once a day. The fish have plenty of things to nibble on in the pond, and the more you feed the fish, the more waste they will produce. The only time you have to feed your koi is when a new pond is installed, because there is no algae for the fish to eat. Also you must feed your koi fish less in the wintertime (depending on your area).

Don't feed the fish when the water is under 55 degrees because the fish metabolism is slowed down and won't be able to process the food.