What do goldfish do when they are over feed?

Answer 1

Very bad things, depending on how much they're overfed. If you accidentally dump in a serving that's roughly 3-5x more than what you normally give them, they can become constipated for a few days. Tank might get more cloudy than normal, but it rarely kills the fish. Dumping in the equivalent of a week's worth of food or more should probably be enough to warrant a water change and/or filter cartridge replacement.

Generally, overfeeding on occasion by accident isn't particularly dangerous. It does much more harm to fish when they are constantly overfed over long periods of time. Apart from reduced water quality, fish can become bloated and sometimes develop swim bladder problems which can easily finish them off if overfed constantly.