So I looked up the ingredients in the FlorinMulti Complete Multi-Nutrient with Iron and Iodine. Although it may be your CO2, it appears more likely that your fertilizer is your "limiting factor". Here are the ingredients:
Potassium - Protein synthesis, water and charge balance, enzyme activation.
Boron - Chlorophyll production, flowering, root growth, cell function.
Carbon - Required for all organic compounds.
Calcium - Cell wall stability and permeability, enzyme activation, cell response to stimuli.
Chlorine - Water and charge balance, photosynthesis.
Copper - Component of enzymes utilized in redox reactions that take place during photosynthesis.
Iron - Required for photosynthesis, component of enzymes utilized in redox reactions.
Magnesium - Component of chlorophyll, enzyme activation.
Manganese - Formation of amino acids, enzyme activation.
Molybdenum and Cobalt - Required for nitrate reduction.
Nickel - Enzyme activation, processing of nitrogenous material.
Sulfur - Component of proteins and the coenzymes that are involved with nutrient utilization and growth.
Zinc - Chlorophyll production, enzyme activation.
Although this fertilizer has a lot of the micro-nutrients needed for plant growth, it is missing two of the major ones.....nitrogen and phosphorus! It does have potassium, magnesium, and calcium. Although you may want to get the pressurized CO2, while researching what you want why not address the fert problem? It will be the least expensive and easiest to correct of the aquarium parameters and you should see results in a week or two. You could try using a more "Balanced" fertilizer like Seachem Flourish Comprehensive, or you could dose some Seachem Flourish Nitrogen and Seachem Flourish Phosphorus in conjunction with your current fertilizer...