I've never had mature fish die (as long as they received proper care in an appropriate environment) unless they succumbed to disease. I have fishies whose ages are well over 6 thousand.

You could always perform a little experiment on the food (you didn't think I was going to tell you exactly how that works, did you?). Put a handful of fishies in one tank and fill another tank with fishies. Add food to both tanks until your fishies are fully fed, and then check them in a few hours. Compare results and you will have your answer.