I grow some in bare bottomed tanks that don't have any lights. I had a crisis and quickly set up 2 tanks to hold some unexpected fish. When I noticed fish in one of the tanks dropping fry I grabbed a handful of java moss and just dropped it into each of the tanks. Since then, both clumps of java moss have been growing nicely with just the light that comes through my north facing window and the bit of light from other tanks in the room. It is also the only java moss that I have that never shows any sign of getting covered in algae. I have come to the conclusion that it will grow unless the room itself is too dark.