i believe that poor water quality will make the fish sick, i left for a hunting trip and told my mom to shake a couple time with the small side and she shook the open side and my fish were at the top gasping for air. so i think yes.
yes and no it depends on the situation with catfish if a large catfish eats a rock or something you overfeed it to make it throw up the rock and make it better
or overfeeding will lead to over wight fish which will make life for them harder more effert to swim and eat and its not good for them in general (plus the other fish make fun of them )
VERY few fish have ever been lost to UNDER-feeding.... I'd hate to count the number of fish I know of personally that have been lost to overfeeding however.