Stem doesn't really look Ludwigia, neither does the stem of the leaves. Ludwigia sphaerocarpa comes closest, but I don't think it's that one.
It look very " Nesaea-ish ", to me. There are several species. Did it go red very fast (as in a few days) after being submerged??
Is the stem "fragile" and "crunchy" or more difficult (= need fingernails or sciccors) to break??
But, (as I allways say, I know)......pictures of leaves are not very good for ID, since leaves look quite different according to environment !!
- if you could let part of the plant grow out of water, it would probaply flower (Ludwigia defenitely would), making ID a lot easier.......
If it's Nesaea, it likes good light and reasonable ferts - and it will grow fast and easy. To be trimmed for development of bushy appearance, though.