Bacopa australis. I haven't seen a more diverse plant yet. If you leave the stalks floating, it grows on the surface rapidly without a problem. If you plant it in the substrate but don't trim it and don't give it too much light, it grows to the surface and then it gets so long, it starts covering the surface. If you give it a blast of light and trim down to one node, it grows in a beautiful carpet on the substrate. It's rather invasive but one of beautiful invasive ones and very good for keeping a tank stable.