In an aquarium, how do you clean the wasted food?

My fish gets stubborn sometimes and will only eat maybe one time a day instead of 2-3 like others should. So the food eventually sinks and goes between it's pebbles. I had previously posted about my fish dying when I cleaned its tank and typically like to clean that grime out. But should I just leave it be since you're suppose to change percentages of its water? Your advice has been awesome, thanks for the feedback.