Could the food be getting into the substrate or trapped under rocks? If your fish are eating that much then it is good but you must be very carefull to not leave uneaten food in the tank as it can quickly foul the water. Have a good check under everything in the tank but if it is clear then you have no worries, though i would cut back a little even if just because of costs, even if your fish do eat that much they certainly dont "need" to.