May be one of the many types of Ludwigia; the aerial root bottom center looks familiar. Leaf sizes can vary a lot depending on growing conditions, especially with poorly fed plants, they tend to get smaller and smaller. Some emergent rotala looks similar to your plant and develops the typical long thin leaves when submerged for a few weeks. Try floating it near a light and see what happens; new growth should be visible in a few days and most stem plants won't fry in good light. Semi-emerged with strong light and ample atmospheric CO2 will get it growing about as well as any other way and give a better idea of what it may be.
An entire world is in your tank.