See if Limnobium spongia fits.

Unfortunately Limnobium spongia is not the plant; though the leaf shape is similar. I should mention that, when growing in water, the leaves usually do not float at the top of the water--the petioles and leaves stick out several inches.

I have no idea of scale, but a possibility is Dichondra carolinensis.

This gives a sense of scale--I think the thickness of the petioles and stems differentiates it from Dichondra carolinensis. It is quite a strong plant.

Then back to Limnobium. Can you take a pic of leaf bottom?

You really can't put too much trust in Internet photos for species that are sold for aquariums, because they are often entirely different from the same species in the wild. Leaves for Limnobium spongia can be floating, or emergent, and plants could easily be found on mud if water depth varies. Since I've never seen L. spongia, I'm not stubborn about the ID :). However, you might find the following link useful, since it provides a detailed description of that plant - Limnobium spongia from FNA.

Underside of the leaf: