Thanks, guys!
fish-n-chips, the bulbs were sold as Nymphaea stellata, Dwarf Lily (but it might be Nuphar stellata, which is apparently the North American species?). From what I've read, Tiger Lotus, N. zenkeri, looks the same but is bigger.
FastFly, yes, that's a blue ramshorn, and ain't she/he a beauty? Cruises around like an ocean liner, with the most beautiful antennae or feelers or whatever those wavy things are. (Can you tell I'm new to snails?) I've read conflicting info about the blue coloration. Some say it's breeding, others say diet. One source said that brown/black shells actually are those colors — but blue shells are translucent, and what you're seeing is the color of the ramshorn's blood!
Anyway, I can only assume that lovely gal/guy came in on a plant. It had reached 1/4" before I ever even noticed it, and now the shell's about 3/4". Good grief, snails grow fast!
I pay a lot more attention to itty-bitty things in the tank now.