I have what I believe is Ludwigia repens, too, under moderate light conditions (28W/10,000K over 10G, in a coralife fixture). It's green on top of leaves, red on bottom of leaves. Sat around and just... "was" (in a Zen fashion) for months and months and months, then decided it liked its situation and has been growing well since Spring, putting out roots and not minding being trimmed and the trimmings being replanted. I like the plant a lot. Mine has a bit rounder leaves than the ones in the pictures though. Probably lots of variations available.