Philosophos, I think your over complicating things a bit. Why would you want to scale back lighting? Because of co2?
Yup, a slight reduction in light because 2wpg of CF over a tank that size is about where it takes good DIY CO2 and excel to keep the algae out. More light isn't so necessary as people think; good spread is.
As for leafzone...
Up until perhaps a couple weeks ago, leafzone had its MSDS listing 10mg/m^3 which is about 10mg/10,000L. It was pure snake oil. I checked today, and they've increased it to a touch over 6% K+. Odds are the density is something like 1.1mg/ml at most, so we can stick with 6% K+ by volume as well.
Good K+ looks something like 20-30ppm in an EI tank, which means with leaf zone you'd need somewhere around 3.5-5ml for every L of column. In a 20 gal tank, the average column is something like 65L without the gravel and a little hardscape. 65*3.5 = 227.5ml/wk. Now, a 16oz (473ml) bottle is something like $5 each; that's $5 every two weeks for a high tech, $5 a month for K+ and lower level Fe for a 20 gal. Compare to dry ferts, or even the fact that TNP+ will supply almost complete nutrition for the same price as leafzone.
It's really not a good buy, and it really isn't going to provide enough nutrients for a 2wpg CF tank. Look at the color of the HC; that's nitrogen deficiency. After K+ is satisfied, guess what else has the same sort of demands that bioload and tap water alone may not provide for?
-Philosophos