You can't put in a higher output fluorescent bulb without upgrading the ballast. <Unless you have a screw in CF with built-in ballast.> Usu. upgrading the light means buying a new fixture. The best bang for the buck would be a Power Compact retrofit kit. Somewhere around 40-50W will give you a solid base for plants, and up to 90W+ if you want to go high tech.
If you want to stick with what you have, Java ferns will be OK. It just will grow VERY slowly. <My Java babies goes from 1/2" to ~2" in 2 years .... and I do have medium light! Yours might be even slower.> You can get a bit more light to the plants by placing it higher in the water, but don't expect miracles with stock lights.