A lot of snails arrive unannounced along with plants or other deliberately purchased livestock. This is my latest arrival, a diminutive species of ramshorn snail.
Enquiries on forums suggest that many aquarium owners have snails of this type. It is in the ramshorn group, small and carries its shell flat with with the ground or the surface it is on.
These snails are not reported to be prolific breeders, so I am letting them hang out for now. The numbers are growing somewhat and they largest snails are now almost 4mm across.
The common names I have heard suggested are mini ramshorn and button ramshorn.
They seem to eat plants a little, particularly my red lotus--but they are otherwise benign.