I have some rotala though and there's a very slight difference in appearance, and this plant grows about 10x faster. I looked it up again and it looks like there's a few kinds of stargrass...the one I guessed it was is "Heteranthera zosterifolia"
there are also different species of rotala, and different varieties of different species. i kept h. zosterifolia. here is what mine looked like:
its the one in the center. r. macrandra on the left, r. wallichi (i believe, its an old pic) on the right. see how the stargrass's leaves are longer and shaped differently?
im telling you, you have a rotala, my guess is garden-variety rotundifolia (if its getting med-lowish light), or r. 'green' if its staying bright green in high light