If I recall, the main reason I've seen is that fish under stress after a water change might not digest their food properly. There isn't any real harm in feeding your fish after a WC but you might want to wait a while afterwards.
Personally, I dont' feed after a WC until the next day just to give their systems a little break. Not sure if that's really necessary but it's worked out well for me.
I don't feed on water changes for a couple of reasons mentioned. Water changes can be very stressful especially when I do a gravel vac. Often times I do a glass cleaning when I change water and the fish gorge themselves on the algae. And I like to give their systems a break. I find this really helps with my fancy goldfish and Betta splendens.
Feeding prior to any disturbance in the tank, which a water change may be, is certainly not good for the fish. Comparable to humans being told not to exercise for 2 hours after a meal. Same principle involved. The fish after eating have to digest the food properly, and this takes considerable energy and several physiological processes are involved. They should be left in peace so everything functions as it should.
Feeding after the W/C is up to you. I don't because some of my fish are not settled enough to eat, and the food would be wasted or if they eat there could be issues like those above. Other fish seem to love the W/C as someone mentioned so there would presumably be no issue. But as it is fine to let fish go a day without food, it is easiest to make that the W/C day to avoid possible problems.
One should never feed fish if you plan on netting them out and moving them. While part of this is the excrement in the container water, there is also the same physiological issue as above.
forum community dedicated to tropical fish owners and enthusiasts. Come join the discussion about species,breeding, health, behavior, aquariums, adopting, care, classifieds, and more! Open to fish, plants and reptiles living in freshwater or saltwater environments.