Long story short: I got four tadpoles that had been on sale as bait. The people had no idea what they were, I'm sure they're ranids, but they could be just about any kind. From what I'm told this particular bait shop orders from a minnow hatchery "somewhere".

Here is a photo of one of them.


They're growing quite well, and seem to be pretty happy in their six gallon tub. If they are non-native I will either build them an appropriate outdoor enclosure, or re-home them with a proper owner. If they are native, I'll probably still look into that.