i make sure the rays are in tip top condition eating whatever they're eating. most effective method is light starvation. sounds bad but its not that bad. usually its only a couple or a few days. after being fed heavily for a period of time they can easily withstand this method and will eat aggressively when you begin offering food again. i mix the old food type with the new in a closed container and feed the new food next day. they should accept it. if they dont then DONT just cave in. starve them another day and try again. keep doing it. remember a ray in good condition can go weeks with no food. i hate doing it but its necessary from time to time.