Snails do not eat poop, they add to the total because they poop too. The fish waste will break down very slowly in a tank but I doubt any of us are willing to wait that long. That means the routine gravel vac that goes with a water change is the way to remove any waste that sits on top of the sand or gravel. The little that works its way down into the substrate will slowly break down and provides a bit of root zone fertilizer to the plants in the tank. If you don't keep plants, you need to plunge the gravel vac well down into the gravel to remove the waste that will otherwise get trapped in the gravel.