ewest, RAD,
I wonder if conditioning is the main reason for the behavior, or something physiological is going on? Many plant eaters have a complex digestive system that may contain microbes to aid digestion and nutrient absorption.
After eating a certain plentiful food for a while, the tilapia's gut may adapt it's microbal culture and digestive enzymes to the plentiful food. If so, it's possible they would have an instinct to avoid sudden changes in diet that would upset their digestion.
That's a lot of "maybe's", and probably overanalyzing, but I wonder about it.
