While taking a walk around the neighborhood today, I noticed that in certain areas where people had shrubs there was a tiny ground covering plant that stayed close to the soil. It seemed to be in shady areas and had almost the appearance and size of H.C. There was no height to it, it was all very close to the soil. Perhaps it is a weed, but looks quite nice and "middle earth"
Wish I knew the name. Wondering if it can be submerged
The first thing that came to my mind was creeping chickweed. It grows in most gardens and easily gets out of control here. Likes a little shade and moisture in the heat of summer. I'm cool and damp enough where you could find it anyplace but in LA I suspect it might get limited to under shrubs in places that might get watered.
The vast majority of terrestrial plants will not survive submersed, although many of our aquatic plants can grow emersed under appropriate conditions. Lysimachia nummularia is one plant that grows as a terrestric groundcover and it will adapt to growing submersed in your tank, however in a very different shape, closely resembling Bacopa, not the ground-hugging growth from your backyard. Bills second image looks somewhat like L.n. but it could be something else.