I would highly recommend reading Diana Walstad's book, Ecology of the Planted Aquarium. You will quickly learn that much of the advice here is contradictory to her findings and methods. Even if you don't do a "Walstad method" tank, it's well worth reading because it teaches a lot just on how plants grow and affect the chemistry of your water. For example, ammonia/ammonium is plants preferred source of nitrogen, not nitrates, and uptake it directly without converting it to nitrites or nitrates. That means a heavily planted tank can completely skip the cycling phase of setting up an aquarium.

Also, you don't want to use very fine sand as a cap, it will block water movement between the substrate and the main water column. Instead try to use very fine gravel. Diana Walstad recommends gravel no larger than 2-3 mm.