According to the article on the first page here, is the author saying that if you intend to use Seachem Flourish, that you should also be using Calcium and Magnesium ferts as well? Are dry ferts OK to use in conjunction with the liquid fert that is Flourish?
Flourish Comprehensive is a trace element mix. You can use any other form of aquatic plant fertilizer with it. You can dose the chemicals, KNO3, KH2PO4, and maybe K2SO4 to provide the nitrogen, potassium and phosphorous that all plants need. Or, you can dose one of the liquid fertilizers, like Seachem Nitrogen, plus Seachem Potassium, plus Seachem Phosphorous. There are a couple of sponsors here who also sell good liquid fertilizers.