Then forget any type of soil. This is creating a much more difficult to manage biological system,and restricting your fish. You cannot have cories unless the substrate is inert soft sand, nothing else. The soil creates bacterial issues which will harm these fish more than upper fish. Soil has serious risks, and until you are much more comfortable in the hobby I would not consider it. It really does not help plants anyway, this is misguided thinking as studies have shown. After one year it is completely useless and has to be replaced; my sand substrates stayed in the tanks for years with no issues, fish or plants.
Believe me, if you really want a "simple" tank as you initially said, do not consider soil.
Are you using tap water? Or something else? Whichever, you must know the GH (general or total hardness) first, and the pH, and it helps to know the KH (carbonate hardness or alkalinity) too as this affects pH.