I completed my first iwagumi style in a Fluval Edge 6 gallon about a month ago using much the same material as you. Using seachem Flourite Black, Flourite black sand, and seiryu stone. Co2 regulator was on back order but plan on installing that today.
Just wanted to share my biggest challenge with this tank so far and that has been with the pH.
My source water (tap) has the following parameters:
6.8 ph straight out of tap settling to 7.4-7.6 after sitting for a couple of days, nitrate 0, KH 3°, GH 3°, TDS 47 and phosphate at 1ppm.
I've been using Seachem Prime for decholrinator as well as Alkaline Buffer, and Acid buffer in attempt to establish a ph of 6.8-7.0. However despite the addition of a large amount of acid buffer daily the ph bounces back to as high as 8.1 within a matter of hours. The last readings I took yesterday having performed a water change the day before:
ph 7.6, KH 3°, GH 12°, TDS 516
Hopefully you won't experience this but wanted to share in case you run into the same thing