I haven’t ever seen a MTS attack a pond snail. I have many tanks with both of them. What I have observed is that pond snails seem to prefer plants and all of the tanks with both types do have plants. MTS are more carnivore than the pond snails, so if the primary source of food is Fish Flake they may out-compete the pond snails. Pond snails also eat more algae and in a super clean aquarium this would also allow them to be overtaken by the MTS population. Could this be what happened in your aquarium, or did you actually see the attack? Are you sure you don’t have assassin snails? Mystery snails will eat flake refuse as well so they provide more competition to the MTS than the pond snails. I do observe the pond snails doing very well in with the mystery snails. Sorry I don’t have a definite answer for you but they seem to coexist very well from what I’ve seen.