Ludwigia Grandulosa


I recently discovered that my single Alternanthera was actually
a Ludwigia Grandulosa, according to Dennerle's book. My tank is
50gal with 80W lighting. Anybody know how many watts needed to
grow this Ludwigia? Listed in Dennerle as full sun plant with
2 sun symbols but how much is full sun? Currently the plant is
not growing well but recently perk-up a little since I removed
the glass cover. Thanks in advance for any suggestion.