Thanks for responding, KaylNeko. I guess I'm talking about pond snails. I thought these common snails were called 'mystery' snails, or mts.
I have some ramshorn snails too which don't multiply as fast as these others. I don't mind them as much, but if the assassin goes after them, oh well! How big of a tank do you have that one assassin can take care of? I've read that one needs an assassin for every fifteen pounds of tank. That would mean five for me - not sure I want that many though. I assume they reproduce and I would eventually get five that way. I'm thinking of just getting two or three to start with.