I use to feed my cardinals live daphnia, frozen bloodworm and tetra colour bits, my largest one was probably just on 2 and a half inches.
I had them in with discus and blue rams and they did really well on 3 to 4 quite small feedings a day with live food once to twice a week.
As long as you are feeding them a well varied diet they will get all the nutrients they need and grow to their full potential. I had mine for about 2 years till the power went out while I was away and the whole tank died
