I know I am not in the majority, but in my 90p, I hardly have 3 to 5L of aqua soil (mixed fluval stratum and Amazonia II). I used a lot of pumice to give height and didn't end up even using 1 bag of aqua soil. Although, my application was different...I knew I wanted to only have hair grass and slow growing mosses/ferns/buce in my tank so I intentionally cut the amount of aqua soil used. I understand it also depends on the idea for your scape and the intended plants....but 18-27L should be more than sufficient?