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.
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.