I've had great success in growing java moss with no artificial light in
a 5 gal guppy tank. It only got some filtered sunlight at certain times
of day. It was the only plant in the tank, it was growing fast, and it
had a bright green colour. Basically the lower 1/3 of the tank was all
java moss. Now I grow java moss in a heavy planted 20 gal tank. It
still grows fast, and creeps along the ground, but the colour is a much
darker green. I use pebbles to hold the moss down, and from there it
spreads around the gravel. It takes it awhile to attach, so its best
not to disturb the substrate at all. I use the same approach with a
java fern. I bured some roots in the gravel (but not the rhizome), and
just left it alone. It then started to attach to a nearby pebble and
gravel. (I have small pebbles scattered everywhere around the plants).
This particular fern started as a tiny plant that came entangled in
java moss. It was awhile before I found it and realized what it was. So
far it grows nicely, growing new leaves. The trick is not to disturb
these plants, which is hard to do in a small tank.