Lumens are the most important - but as long as you stick to fluorescent
lighting you can just think of this as watts since the wattage of a
bulb is usually easier to find. And yes, your tank probably could use
some more light.

There are two solutions for you to get healthy growing plants in your
situation. The cheapest is to change your plants. Anubias and java fern
will both do fine in 1.45 watts per gallon as will several other low
light plants. The other solution is to get your lighting up to between
2 and 4 watts per gallon. Ways of doing this are to either replace your
lighting completely with something in the 120 - 200 watt range or to
just add a second fixture if the top of your tank has room for it.

-Daniel