The species of Microsorum listed for Vietnam are:But this one has wavy and needle leaves. I am told that this plant has been found at the central of Vietnam years ago
<"Looks promising">, but I have no idea whether you can grow it submerged.Microsorum fortunei
It doesn't look like it was a Ludwigia (Onagraceae) type flower, I still think it is a plant from the Lythraceae.Talk again today and it looks like Ludwigia inclinata
That looks promising. I can't really tell from the photos but your plant looks to have a short stalk ("pedicel") to the flower, while in Rotala densiflora it looks to be stalkless ("sessile").Search a bit more and looks similar to Rotala densiflora
That <"looks closest">, stalked flower (that is the pedicellata bit).Nesaea pedicellata
Hi AnhBui, Looks like java fern to me 🙂