Blue and pink ramshorn snails. Not sure what the other two colors are called.
Like other snails that reproduce in freshwater, they don't overpopulate unless overfed. They are great for monitoring water quality. Even if you think everything is a-okay, if they travel en masse to the surface and remain there could be something wrong.
I wish I could help but my snail tank has, what I strongly suspect, is planaria worms and the dang things are wiping out my ramshons, even if they weren't I'd not send the snails to someone and risk also sending the worms. My bettas seem to be picking off the ramshorns in my other tanks so I don't have many left in those tanks.