Baymule
Herd Master
Do be aware that snails love ponds also and are a carrier for liver flukes. Wet areas are prime for liver flukes. If it is safe for your ducks to free range, they will eat the snails.
http://www.beefmagazine.com/health/liver-flukes-expand-range-26-states
http://www.thecattlesite.com/diseaseinfo/182/liver-fluke/
http://www.beefmagazine.com/health/liver-flukes-expand-range-26-states
http://www.thecattlesite.com/diseaseinfo/182/liver-fluke/