Part of the problem could be that you have 15 inch lights on a tank that is 48 inches long.
I have
Dual-Lamp T5-HO | Aquatic Life. I would suggest this is you are serious about a planted tank. It is not cheap. I think that I paid $120 for my 36 inch one at a specialty aquarium store. Also, you have to make sure that your top is clear glass that won't block light.
I think my model came with a Roseate (?) bulb but, like I said, I use a 6700K and a 10,000k bulb. I do not get 2 watts per gallon but my plants grow like mad. I have mine on a timer that comes on at 4PM and off at 12AM.
If you're using ferts but not the proper lights, the plants cannot use the nutrients properly.
If that is too much $$ for you, go to Home Depot and get the a few metal wok lamps and c=some CFL daylight spectrum bulbs. That might also help.