Here is an option:
you have free snails.
you don't know how many exactly, or what kind.
Even without tons of food they probably aren't going to disappear.
You don't have fish yet.
I don't know how much work you've put into scaping and planting, but you could always put a jar of water on the window sill, put a handful of the snails you find into it. Then re-start the tank.
That would kill off all the ones in the tank, and then the ones in the jar you could grow to see what species they are, and decide later if you want free snails or not.
There may even be a way to treat the water for the snails, without damaging your tank. Like maybe if you take the plants out and pour a lot of H2O2 (hydrogen peroxide) in, it would kill the snails, but after a few days it would all break down into water and oxygen, and then you could put the plants back in after you dip them in a bleach solution?