It's been my experience with pond snails that they'll go thru a population boom when first introduced to a tank even in the face of fairly limited food. Even if you aren't over-feeding (it's happened in tank swhere I haven't been feeding at all), there's always food in a planted tank in the form of decaying plant material.
If you want to keep some in your tank, but limit their numbers, leave a few mature snails and concentrate your efforts on manually removing the smaller ones. Whether it's a hormonal inhbitor, slower reproduction rates in older adults or a few big ones out-compete hordes of small ones, I've consistently had better results this way.
Last edited by Knotyoureality; 01-13-2014 at 04:15 PM.