Snails or snail eggs actually hitch ride on live plants. Whether you feed allot or just enough, snails WILL multiply. Snail killer meds usually don't work. I even put a concentrated dose on snails that went above the water line and all I did was stunn them and make them fall to the substrate. Maybe I just gave them a nice buzz. Eventually, they got up and started crawling around and multiplying again.
You can remove them by placing a piece of lettuce overnight in the bottom of the tank, they will feed on it, so the next morning, you can get rid of it. Unfortunately, snails will still continue to multiply. One other method I used was I fed them to my fish. Fish cannot eat them with the shells, so if you lightly tap on the snails until the shells break apart, the fish will be able to eat them. Guppies and mollies are grazers so they will eventually eat them. If they don't, suck up the snail bodies with a turkey baster and then squirt it in the tank, so the snail body will fall. Fish usually go after moving things in the water, since they figure it's food.