Assassin snails are only capable of eating 2-3 small snails a week, standard pond snails are fine, their numbers are easily controlled and they're a very good indicator of whether you're over feeding or not, if you suddenly find that you have a massive boom in snail population in your tank, then that would indicate that there is a lot of decaying waste lying in the bottom of the tank, this will more than likely be due to left over food, but decaying plant matter could be a source also. Pond snails are good at cleaning up soft algae's on the walls of the tank and on plants. If you want to get rid of them without picking them out, take a slice of cucumber, weight it so it sinks and leave it for a day, then pick it out the next morning, the snails should be feeding on the cucumber, so you can remove quite a few at once.