I am looking at my plant book and came across this one. Sounds like the
plant you have. Here is what the book said.

Ludwigia repens
water primrose

family : Onagracae
range : tropical n. America and c. America
appearance : Stem plant, about 20 in (50 cm) tall. Leaves decus-sate,
short-stemmed, roundish to broad-ovate; upper side olivegreen,
underside reddish to deep red/purple. Color is dependant on light, in
week light the plant remains pale. In emersed culture, small flowers
with yellow petals may form.
care : Tolerates coolish water better than overly warm, needs nutrient
rich bottom and regular additions of fertilizer after every water
change. Branches prolificly, so be sure to leave enough room at the
sides when planting.
Light : 50 w per 25 g (100L)
water : 68* - 86*F (20 -30 C)
2* - 15 * dCH; pH 5.5 - 7.5
propagation : by cuttings
placement : use in groups in the middle and along the sides.


Hope this helped.
Vicki