It grows like a weed in my tank and I can't figure out what it is.
Yeah, that's the 'green'. Nearly certain to be a form of rotundifolia.Conditions are ~30 PAR, daily excel, ~1/3 EI. I forgot to mention that it tends to grow sideways and throws out a lot of aerial roots. It prefers to entwine itself with my other plants instead of forming a nice vertical bush.