I'm a huge fan of Assassin Snails, having rid me of a huge pond and ramshorn snail infestation. That said, they will only deal with the smaller MTS, so I suggest that you remove the larger ones by hand, and add 6 Assassins. If they themselves breed, the offspring will have some value, when they get to about 6 months, and they don't breed as wildly as pest snails.