Originally Posted by @marko@
looks more like a rotala, rotala green perhaps, than stargrass. stargrass has much larger leaves comparing to the stem, and the leaves end in a sharp point, not a rounded edge.
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"