Yeah i can understand when money is tight.. Balancing lighting and fertilization with a good substrate are all important factors in the health of your plants, especially for those fast growing stem plants. I'm not an expert in lighting but if i were in your shoes, i'd start with upgrading the fixture. Perhaps someone here can give you a recommendation. But from what I've learned, the whole watts per gallon measurement only applies to standard 'NO' (normal output) bulbs. When you step up to T5 HO bulbs, the watts per gallon isn't the same due to the higher quality reflectors and so forth.
With that said, you can buy a 24w T5 HO fixture from Catalina Lighting for about $60.
http://www.catalinaaquarium.com/store/product_info.php?cPath=71_198_200&products_id=1834 as one example. I'm sure they're might be other options, you'll just have to research and see what's best for your needs. On the other hand, there's the HO LED options now too.. Finnex Ray II 24"daylight fixture would give you excellent lighting for under $100.
http://www.amazon.com/gp/aw/d/B008RMFDEG/ref=mp_s_a_2?pi=SL75&qid=1348517193&sr=8-2
The advantage of LED's is that you don't have to change the bulbs every year or so. Plus power consumption is significantly less for the amount of light you get, which the fixture pays for itself over time. Just something to consider.
As for using gravel as a substrate. Its definitely not the best choice for a planted tank. Trust me, i know first hand. I just recently switched out my standard gravel on my 12g nano cube to Caribsea Floramax and my plants love it. I can tell just by how the root systems are when i pull a stem out. Also by how the dwarf hairgrass roots are by the front glass. I would recommend changing the substrate. I got my 12lbs bag at Petsmart for $10. You can also use Eco Complete or Fluorite. However, using just gravel is not totally out of the question. You can get some root tabs and stick then in the gravel to help your plants out.
I also recommend injecting co2. You can setup a DIY co2 system for very cheap. There's how to videos on YouTube, how to's in the forums, etc. I just set mine up about a couple months ago. It's like the 'holy grail' (as one AA member here put it) of amazing plant growth (when mixed with proper lighting and ferts).
Lastly, you need to consider a way of supplementing your plants with nutrients. You can do the root tabs as i mentioned, along with a bottle of Seachem Flourish and Excel. You can supplement some Iron and Potash with API Leaf Zone. Or, invest in some dry ferts you mix up and dose using a popular method of dosing by planted tank enthusiasts (see EI dosing vs. PPS-pro). Going dry ferts is a little more challenging but saves you money in the long run. But to keep things simple, I'd probably start off with Flourish, Excel, and Leaf Zone.
Sorry if my response was super long. Just trying to be thorough and cover all the bases I can. LMK, if i can help. Good luck!
