yep. i forget what the ratio is, but it was something like 40, or 60% of the food fish eat comes out undigested. it just gets put back in the water column and breaks up into tiny particles.
it's an excellent food source actually.
the best thing you can do is be diligent with your nutrient removal. in a closed system you would be surprised how fast nutrients get excessive.
remember, when you feed the tank, the food really doesn't disappear. it just gets changed.
keep the water clean and your fish and corals will thank you.