One of our GSAS members was in San Francisco and visited the ADA distributor there, Aqua Forest Aquarium. He also picked up a 'mini' Hydrocotyle that was being sold as sp. 'Japan'. Here is what he wrote in our GSAS member chat:
I bought it as Hydrocotyle sp. Japan in San Francisco but I realize it is H. tripartita.
You might want to see if that is what you have as well.