That looks very much to me like Rotala Rotundifolia. Mine is in an EI tank with 3.5WPG, and is doing exactly the same.
Apparently, it is red when it is emergent and green with a finer, more delicate leaf when it is submerged. If you want to keep a level of redness, allow some to float at the top for a few days and replant. Keep repeating this process.