If your jack dempsey eats all kinds of foods, why are you feeding it live fish?
Apart from the cruelty aspect, it isn't especially healthy for a pet fish to get hooked on live feeder fish. If at all possible, with aquarium fish you want to adapt them to a diet of flake or pellets. Only that way can you ensure they get a balanced diet. Add frozen foods like bloodworms for variety, and you're laughing.
If you simply like watching one fish eat another fish, then that's a bit sad really.
Cheers,
Neale