Plants need...
1) Light - if you've not got enough light, no amount of CO2 or fertilisers will help, intensity, (appropriate to the plant), quality, (appropriate spectrum), and duration, (10-12 hours neither more nor less)...
2) CO2, If you got enough light for active photosynthesis, CO2 is the next limiting factor. In a tank that size, assuming you have a decent fish load, CO2 is probably adequate. Adding CO2 if there is not enough light will acheive nothing...
3) Macro and Micro nutrients - the fish usually provide enough of these in all but the lightest loaded tanks.