Pond fish, like koi and goldfish, can go a surprisingly long time between feedings. The exact time depends on how old the fish is, how big it is, and what it eats. In general, carnivorous fish have tendencies to skip a day of meals here or there, as their meals are generally very hearty and filling. Plant-based diets are less filling because plants are harder to digest, resulting in less total food intake. It also makes the fish need to eat more often to have their nutritional needs met. Large and old fish generally have more of a fat reserve. This means that they can go without food longer, as they can rely on their fat stores if they need them. Most of the time, though, fish are pretty good at scavenging and finding their own food in the event that you forget to feed them.