What type of light is it? 12 watts is not a normal flourescent tube, so I thought I'd ask, since it can make a difference.
If it is normal output, the lighting is so low in there you may have trouble even with java moss. If there is any growth, it will be extremely slow (I mean, you may stare at it for months and see no difference). And it may brown and actually thin out. I'm not saying this is definate, each tank to its own. I do believe it is probable that you will not see much if any growth.
The watt per gallon "rule" degrades as you get into very small or very large tanks. Smaller tanks need more watts per gallon. I think the watts per gallon rule holds true for tanks around 50-80 gallons.
