I will deal with Flourish in a moment, first just to say that light is the primary factor in aquarium plants, and the nutrients must be balanced with the light intensity.
Flourish Comprehensive Supplement is a complete plant nutrient additive [I'll explain the exceptions in a moment]. It is, as the name suggests, a comprehensive or complete "supplement" to the nutrients already present from water changes and fish foods. It is not intended for high-tech planted tanks where mega light and diffused CO2 are part of the balance which thus requires larger influxes of nutrients. But in the natural (low-tech) normal primarily-fish aquarium that has plants, it is ideal.
For this purpose, it is "complete," but it does not contain three nutrients, namely oxygen, hydrogen and carbon because these are naturally occurring in an aquarium. The remaining 14 nutrients are also in a specific proportion to each other, as botanical studies have determined. The "hard" minerals calcium and magnesium are minimal because most water supplies have these minerals in them. For those of us with very soft source water, or using RO water alone, it can sometimes be necessary to add these two minerals to compensate, but this depends upon other factors such as the plant species, numbers, and fish species/load.
The proportional balance is more important than you might think. Studies have determined that an exzess of some nutrients--iron is one of these--can actually cause plants to shut down assimilation of other essential nutrients. The proportional balance of nutrients in Flourish Comprehensive Supplement avoids this problem. But it means that other nutrients--excepting calcium and magnesium in some circumstances--should never be added in addition to Flourish Comprehensive.
Brightwell Aquatics' FlorinMulti is basically identical with respect to the above. I just looked up Easy Green, and it has 11 nutrients, and calcium is not one of them, so from that aspect the Flourish and FlorinMulti are better, but if you have moderately hard water calcium is or should be sufficient anyway. But from my searches, I would go with either Flourish Comprehensive or FlorinMulti. I have been using Flourish Comprehensive for more than a decade now, with excellent results, and I have very soft water (zero GH/KH). If I could get FlorinMulti locally I would probably give it a try, but I am not going to order it in when FC is working OK.
