This is very interesting. I have discovered a very tiny ramshorn snail-like critter in my 10g, and they never get larger than about 2 mm if that. I believe mine arrived from dried oak leaves collected in the back garden. The 10g tank in which they appeared has had no new fish for two years (except Farlowella fry from my 70g and there are no ramshorn snails in there), no new plants or moved plants from other tanks in over three years. The only addition has been the oak leaves collected in the fall. I dried them on paper towels but did not wash them, so I have presumed the snails are terrestrial and not surviving in water. ?? There are also pond snails and Malaysian Livebearing snails in this tank.
I have managed to photograph the "ramshorns" by accident. Here's a photo of an oak leaf holding a 2-week old pygmy cory fry, and in the lower right you can see three of these snails. Initially they are very bright red. They never get larger than these in the photo.
Byron.