not true. cories are diurnal. however, they will feed after dark, especially if they get used to feeding that way. mine get really excited every evening after the sun goes down. try breaking up the wafers and dispersing the pieces all over the tank instead of dropping them in one place. once the group of cories get used to being in your tank, they will be a little more rambunctious about eating. mine will steal wafers away from my congo tetras!