My assassin snails spend their time jet-setting from tank to tank. I like snails and have generally found it easy to keep populations in check, but occasionally I slip up or, as with my cherries, have a small tank that I want to keep snail free. That's where the assassins get called in.
My 3g shrimp tank had new substrate and I checked over the plants as I transfered them, but I knew I'd have hitchikers, so a couple assassins were fished out of the planted vase they'd been living in and dumped in. I spotted a few snails the first few days, but after a week the only critters in were the assassins. At which point I fished them out and dispatched one each to planted vases that'd gotten overrun thanks to my being a bit heavy-handed with the feeding and lax on cleaning. Now they're both living the good life in my 37g nibbling away at excess MTS with the third exiled to a 2.5g to chill--cause I don't want TOO much rambo-action in the 37g and I know they'll be more difficult to locate there so it'll take time to extract 'em.
One trick I've found with pond snails, is to concentrate on removing the small snails first. Repeated tests with hands-on experimentation has pretty much shown me that the drive to reproduce is strongest in the younger snails. Remove the occasional adult, but concentrate on taking out the bevy of young ones to keep them from reaching reproductive age. I suspect the older ones also help deplete resources---better ONE snail that's eating 70% of the available food and will only rarely lay eggs, than a herd of small snails each requiring much less food each who ALL lay eggs as often as possible. The only tanks/planted vases where I get a bunch of new young snails successfully hatching out and surviving are the ones where I purposefully remove all the largest/adult snails. Invariably I then get a population BOOM. Reverse that process and, even with lots of egg masses on the floaters, I go months without seeing a new hatching.