12-18-2009, 12:07 PM
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First, Flourish Excel is not a fertilizer comparable to Nutrafin or Flourish Comprehensive or other similar products. It is a liquid carbon supplement. I have never used it, and unless I were to have a terrible attack of brush algae, probably never will. I have no reason to use it, as the fish in my aquaria produce sufficient CO2 for the plants; they wouldn't be thriving if this were not the case.
Liquid fertilizers like Flourish Comprehensive and Nutrafin supply the minerals that plants need because these may not all be available through fishfood, tap water, etc.
Carbon is required regularly by plants. If one feels there is insufficient CO2 in the aquarium, and resorts to using Excel, it should presumably be fairly regular. It is comparable to diffused CO2, as Seachem explain on their website. And it is my understanding that those with CO2 use it daily, not once a week or once in two weeks. I wouldn't think that would accomplish anything beneficial--except perhaps for algae.
Liquid fertilization for minerals is normally sufficient once or twice a week, depending upon the balance with light and CO2.