Ms.Bubbles said:I would say you need about 80 watts of fluorescent lighting to grow low-light plants such as java moss/ fern over a 55 gallon.
debaric said:you can grow java moss and java fern in less than 1 wpg. It wont die if you grow it in 0.5 wpg. It will just look like its not growing at all but it will stay good looking.
Ms.Bubbles said:A 20 gallon tank needs about 30 watts of light over it to grow most of the low-light plants, so you need to increase your lighting.
Also, hopefully, the plants you've got aren't medium or high-light plants, or else you will need to get much more lighting--more like 40-60 watts.