My mistake, though I've seen Japan grow nearly that big.
It's most likely Marsilea. There are 5 species at least, most if not all invasive in some places. Far as I know, all are marginal and don't grow well submerged for any length of time. It has black rhizomes, unlike pennywort.
Not that common in stores, I sometimes see M. minuta, a tiny leafed form that grows only a couple of inches tall and is supposed to be a carpet plant, but I've never seen it grow well that way. Or quadrifolia, which can get about 6-8 inches high and won't grow submerged for very long at all. Happiest in water a few inches deep. It's a European import.
It does resemble Hydrocotyle tripartite Japan, which is a variant of one of the dwarf species of Hydrocotyle.