Dude - your problem sounds so familiar. I have a 55 gallon and have gone through 200 dollars in plants. More than the fish cost - all in the effort to give the fish a close to natural biosphere. Then I bought the Hagen Glo system at 48 inches... T5 power glo and life-glo Ho lamps with reflector. Expensive but I thought I wanted the plants to grow. And I still want the plants to grow! So I added CO2 to the biosphere and here is how...
One Juicy juice bottle, a bottle of yeast for bread, and sugar. I bought 8' of tubing and drilled a hole smaller than the tubing by 1/16" mixed the sugar ( fuel ) and the yeast micro-organism. I learned from a LFS in Atlanta Ga to place the tubing under rock or driftwood for natural diffusion and well the plants are growing and flowering. Caution - PH drops significantly when introducing CO2 in the tank enviroment so add shell - crushed shell or coral - or PH enhancing chemicals or rock. The natural diffuser works like crazy!
Also I had green spotted Algae exlode all over my plants and glass when I added the new lamp system. I moved the lamps higher from the tank with the glo-system brackets supplied and changed water weekly for 3 weeks 1/3 the tank capacity. The algae is still there but the plants are growing and flowering and the algae has pretty much been diminished. It is the blue light ( not the red light power-glo lamp ) and nitrate levels that allow the algae to grow - so it seems to me. Vacuum your rock to do water changes and note how your nitrate levels will drop as will the amount of algae.
Write back if you have any questions and great luck.