u/Ala-Kazoo

I'm looking at setting up a fresh water 21 gallon long for pea puffers, (35.43x11.81x11.81) for the first time and I'm a bit conflicted on what kind of substrate to use because of how shallow the tank is. I'm more partial to having a heavily planted tank for the puffers, so my usual set up is 3 layers, which consist of soil, sand/gravel mix, capped with gravel, sand or black diamond black diamond blasting sand, which is at least 2 inches thick. If I use the same layering in the 21 long, this maybe leaves about 8 1/2 inches of tank real estate, in height, and I'm wondering if this would be enough depth for pea puffers or if this is too shallow and if I should opt for a single substrate instead.?