The primary nutrients plants build tissue from are nitrogen (nitrates), potassium, and phosphorous (phosphate). With the high light you have you need to be dosing potassium nitrate and mono potassium phosphate, or, if you are using Seachem products, Flourish nitrogen, fluorish potassium, and flourish phosphorous. Also, with the amount of light I think you have, you really need to use CO2, not "liquid CO2", which isn't CO2, but is a chemical that plants can obtain carbon from, and which is not nearly as effective as real CO2.
If you can't run your light with just one bulb, buy some gray fiberglass window screen, and put a layer of that between the light and the top of the tank. That will reduce the light intensity about 40%, which should make it a lot easier for you to get by with just Seachem Excel or the equivalent product by other manufacturers.