Small snails seem happy enough, after a bit of research they're most likely hermaphroditic bladder snails and were introduced via aquatic plants from the garden centre, either juvenile snails or eggs hitched a ride on plants to our water planter. As the term hermaphroditic states - they have both male and female reproductive organs and can even self-fertilize so one single snail can start a population but they don't seem to be causing a problem or overpopulating, if anything they're a little clean up crew that eats any algae or dead matter. Seeing one move across the surface of the water while upside down (hanging to the underside of the water surface) is kinda funny.