Id put in maybe a liter or 2 of water (1cm of water in the tank) and put in a couple handfuls of substrate at the base just to help, the other substrate to them cover it which will have the main contact with your water allowing for the buffering effects (and the colour difference if there is one wont be noticed).
I currently have a small pipe heater in the current tank that i can use for the new one, that technically makes my new project cheaper because i now don't need 2. can i get dimensions of this thank fully assembled including the top cover just so i can determine if i need to adapt my shelving. i don't mind poking out a corner of the tank from the shelf so i can do water changes and i can lift the tank onto another surface for heavy maintenance if necessary. Im really just interested if there is space for feeding without moving the tank.
I add 5-6cm thick layer of soil ... ADA is ok to use.. should give you a ph at mid 6 range... I find the more you add the longer the soil lasts and the stronger base of the water their is as their more soil to Buffer the water... the ph will no go lower will say at mid 6 on ada but will last longer the more you add to a tank so less time changing it over next time