Welcome to the forum.
I have moved this thread to the Aquarium Plants section since that is the main subject of your post.
I have several planted tanks running at present [photos of some are under the "Aquariums" tab below my name on the left] and planted tanks have been my thing for 20+ years. The only substrate method I have not tried yet is dirt. Primarily because the benefit does not in my mind make up for the issues. Depending what sort of planted tank you want, you can achieve it with plain sand or plain fine gravel. One basic method is outlined in my articles at the head of the Aquarium Plants section entitled "A Basic Approach to the Natural Planted Aquarium" that you might like to check out. For the present, I'll move on to other issues.
And first is the fish. I am assuming you have decided on a planted tank as opposed to a biotope which would have few if any substrate-rooted plants in keeping with the natural habitat of Pterophyllum scalare. However, they do move into the flooded forest for half the year, so a planted tank is still "authentic." These fish do not appreciate bright overhead light, so minimum lighting works best. Sufficient for the needs of the selected plants, plus some floating plants, and this should be fine.
A substrate of sand would be authentic, and here you could use what I now use, Quikrete Play Sand from Home Depot/Lowe's, which is very inexpensive--two 50-pound bags will do your 90g with some to spare.
Filtration can be minimal too, as these fish do not like strong water currents, and neither do the plants. For a 90g i would go with a single canister rated to the tank size. This is what i have on my three largest tanks.
I'm sure you'll have more questions.
Edit: Decided not to move this thread, as various topics are being covered so it might as well remain where it is.