You certainly could get a higher watt bulb, but some fixtures are only rated for a certain wattage so make sure before you change bulbs. I am currently growing Blyxa and HM in a 5.5 gallon with 27 watts of compact fluorescent and they are both thriving, if that helps you gauge how much light you may need... Good luck, I'm sure it will look great!
Adding more CO2 will help any plantd tank. HC is especially picky and I would say it is almost a necessity.
You can also get a 27w 6500k CF desk lamp at Home Depot. Its around 20 dollars and I have only heard good things about that lamp. Remember that adding more light without increasing the available nutrients, ie ferts, for the plants will only give you an algae farm. Also without the right amount of CO2 or ecel you will also be limiting the plants and algae will be more than happy to make up te difference.
The tank has been running for a few months now and I still have not had any algae problems.
The lamp I have can actually handle up to a 60w bulb (was bought at Home Depot BTW).
I rather like the low light, anything else would be too much for my small room. I do have dry ferts, but I'm still trying to figure out exactly how much to dose in such a small tank before actually adding any.
What is the gh, kh, ph and co2 of the tank. There is always Co2 in the water.
How about just trying a different plant. Four leaf clover would probably work. I have it growing in my low tech 29 gallon tank. I had 65 watt of PC bulb and 20 watt T8 bulb. Now just went down to 2 - 20 T8 bulbs. No Co2, dosed with flourish comprehensive daily and sodium bicarb when doing water changes.