They actually might not be able to survive as young after hatching. If you noticed then starting to grow then they might have adapted to your water conditions. But considering how slow snails move I can't see them making a journey down the river to the sea, lay eggs and make their way back up the river. But who knows, maybe they live to be a hundred and time is on their side
. If it becomes a problem you can get snail eaters. Throw a fiddler crab in there and it'll make escargo niblets out of them