It's Pogostemon erectum. It matches a good botanical description and herbarium specimens I've found. I've seen preserved Rotala verticillaris and it's easy to see why they were confused. The inflorescences aren't at all the same though.
Please see here:
http://www.aquaticplantcentral.com/forumapc/plant-id/58042-rotala-verticillaris-nope.html
Please see here:
http://www.aquaticplantcentral.com/forumapc/plant-id/58042-rotala-verticillaris-nope.html