I too have never used Tropica products, though they have a good reputation. The soil would be less of a problem if you just stay with soft water fish, as they will appreciate a lower GH and pH. Avoid livebearers, they must have harder water than here. But another issue is fish...if you wanted cories or loaches, this substrate may prove to be very problematical. Cories must have soft sand, loaches prefer this. The "nutrient" issues in plant substrates can also cause problems for fish that live on the substrate, involving bacteria and such. A 20g tank with no fish can easily be re-set with a fine sand substrate, that is your call. I did have a plant substrate some years back, and after two years removed it because it was too restricting with as far as I cold see no plant benefits either.
I am inclined to think the green botches on the plants is related to the fertilizers and not light. As I said previously, never add phosphates. The Tropica site says it contains "iron, manganese and vital micro nutrients" and "does not contain nitrogen and phosphor" which is good, but it does not say which "vital micro nutrients" and iron and manganese are themselves micro nutrients. But on its own, it might be OK, so use it until it is gone and then consider one of the better (in my view) products. I can list these if asked.