My lfs grows it with natural light. It completely covers the top of the aquarium they grow it in. I grow it with natural light and a 40 watt bulb.
I also use aqueon plant food at a half dosage.
All in all I have both melt, rot, and propagation. Varied mixed results. They do tend to drop their roots on me, and my baby snails like to munch on it. Other than that it grows just fine.
I'd recommend a Lilly type plant over the water lettuce though, just personal preference.
There are 2 of us
Naldari and Naladari