08-08-2010, 04:42 PM
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You've asked about Excel so I'll answer. It continues what has already been mentioned by zof and Johnny, but I'll expand a bit.
If you had a look at that article, or read the rest of the series, the section on nutrients explained how plants need 17 nutrients in proportion, plus light. These together have to be in balance. Too much of any one, or too little, and the plant growth can be negatively affected.
The first premise in a low-tech or natural setup is minimal light, as little as possible, just sufficient for the plants' to photosynthesize. Then you balance that with adequate nutrients. Carbon is one nutrient, nitrogen is another; both these come from the fish and biological processes, and in most aquaria there is sufficient to balance the lower light. The mineral nutrients we add via a comprehensive fertilizer, Seachem's Flourish Comprehensive Supplement for the Planted Aquarium is probably the best out there at present. Once a week may be sufficient for your setup, or twice; it takes a few weeks to find that "balance" in each aquarium, which depends upon the number and type of fish, the plants, water volume, and source water.
To get back to the Excel, this is a carbon supplement. So it is adding additional carbon to the system. Once you start increasing any single nutrient, you offset the balance. Which means you have to increase the light and other nutrients to create a new balance. Taking this to the farthest extreme, there are the high-tech planted setups which use diffused CO2 to up the carbon. To balance, these must have 3-4 times as much light, plus an injection of liquid fertilizers daily in most cases, and substrate nutrients are almost mandatory. This is fine, except the more stuff you add, the more you are "pushing" things. My approach over 20 years has been to keep it simple; as minimal as possible to achieve reasonable plant growth. I let nature do most of the work--well, all of it actually; I only provide some light and nutrients to balance. Plant growth may be slower (depending upon how you view this), but it will be steady, the plants will be healthy--and perhaps most importantly, the fish are not being bombarded with stuff every day.