Is that 15 watts of fluorescent light or is that incandescent light? It makes a huge difference. It's hard to quantify light with commonly available information. Watts are a measure of the power consumed. It doesn't really specify how much of that power is actually converted into light vs. heat, noise, etc or how powerful and plentiful those photons really are. This is what causes so much confusion in aquarium lighting.
I'm going to assume you're running a fluorescent light because your sword plant is surviving. Even so, you're still in the low-light range. Anubias, java moss, java fern, and anacharis are all low-light plants. Amazon swords can survive at lower light levels, but they won't grow much. Be prepared to remove that sword plant. I have two that are busting out of my 29g.
If you really want your plants to take off, upgrade your lighting. T5HO fluorescents seem to be the best option right now, but you might need CO2
to prevent algae. The simplest option would be to add another light fixture exactly like the one you've got to double your light.