I was able to train my fish into accepting mice in their diet at around 3 inches on small, live pinkies.
tip #1 no need to really freeze the mouse to slow em down unless you are sure this will attract their attention. I simply took a piece of butchers string, and tied a small piece of bloody steak to him, then let him rest his little forearms on a piece of balsa wood.
tip #2 the reason the balsa wood kicks ass, is that it acts like a life raft for the guy and he does a lot less work to keep afloat. this = more time for him to kick his little legs and attract their attention.
tip #3 once you actually have a system down, stick with it until it works consistently every time, otherwise the fish start all over from scratch trying to learn new behaviors if you toss it up too much. which = you waiting forever to HOPEFULLY watch them eat the luckless bastard = boring
tip #4 turn off the powerhead (s) if you have any. mouse fur is a bitch to clean out of them.