Koi eat a lot, Goldfish do too, but Goldfish are better foragers.

If you took two identical ponds and you neglected to feed both ponds equally, the population in the Koi pond would die out faster than the population in the Goldfish pond. Part of the reason is that Koi eat more, so they starve faster. Part of the reason is that Goldfish will find food anywhere, including swimming prey like rotifers, fish fry and insect larva. Koi on the other hand tend not to identify or attack small prey like that. Their usual foraging method is bottom sifting. And if the pond has no aggregate or mud on the bottom, there will be no natural forage. I think most people would be very surprised by the amazing amount of live, squishy and crunchy food that can be found living in the gravel of a properly maintained Aquascape pond. Sometime if you're ever bored, siphon thirty gallons of water out of that gravel and put it in a plastic sweater box. Let the sediments settle and then look at the writhing life making tracks in the settled sediments. That's Koi food, friends.

 Overfeeding

The majority of hobbyists, if they were to make a feeding mistake, would overfeed. This is because the feeding process is arguably the best time you have with your fish. At feeding time, Koi come up to eat so you can see them, and anyone with a maternal instinct will thrill to watch their favorite fish engulf food with such Koi-ish zeal. Overfeeding is engaged anytime the fish are eating more than they need. So, for a fish farmer with a mud pond full of small fish he's trying to grow, he would feed about five to seven times per day. That would not be overfeeding. But in a typical ornamental pond with a reasonable collection of fish, that feeding rate would be excessive and the wastes therefrom would strain the limits of what can be biological reduced. Means: You water quality will decline.

 Fish that are overfed in typical ornamental pond facilities will eventually develop ponderous bellies and begin to look a little bit like tadpoles, with the big body and the wispy tail. That is not a terminal event for the fish but the impact on the liver and other internal organs can and will be severe. Farmed fish that have much more room in lakes or large ponds can be fed considerably and they will not develop as much obesity as reguar backyard pond fish.

 Fish should be fed no more than three times per day. In cooler water they should only be fed once per day, if that. In much warmer water, three times per day is not 'crazy' however, you have to be wary of bacterial blooms (cloudy water and low oxygen levels) if you feed heavy and there's a lot of waste.

 Fish should be fed for about five minutes per feeding. If they don't come up and eat voraciously, they are telling you that they are too cold, or too warm, feed light. If they come up and "hit hard" you can sprinkle food on the water for five minutes as long as there are fish there to carry it off. Pretend it's a game, NEVER LET IT FLOAT. So, feed the fish as they approach and let them carry it all away, leaving none to float into the skimmer or filter.

 Underfeeding

Sometimes a person is very busy and they may neglect to feed the fish every day. This only impacts certain groups of fish. The very large fish in summer will rapidly lose weight as their metabolism is working optimally but there aren't enough calories for their big bodies. Very small fish which need to grow will also be stunted or die. Fish in ponds with natural forage and some plant material will help themselves to nature's bounty. Fish in indoor or liner ponds without additional access to forage will lose condition. If you fish are growing about a half-inch per month, you're feeding enough.

 If you have a Goldfish or Koi that is NOT growing a half-inch per month you are either underfeeding, you are keeping them in too small facilities, or the food is not adequate to push growth. Many fish grow one inch per month. Even relatively slow growing Ranchu's can be pushed to one inch per month in large facilities and fed fresh proteins.

 Signs of underfeeding include: Heads wider than bodies, slightly sunken eyes, a kink at the base of the tail, poor color, thinness, trailing white stools, and inactivity.