Reading through this thread, a cojple of things need clarifying.
The GH, KH and pH are connected/related, but they behave differently. First the GH which is the measurement of "hard" minerals (calcium and magnesium primarily) dissolved in the water. The GH of the source water (tap) is not going to change much in an aquarium. It is easy to increase the GH by using calcareous substances (rock, substrate, etc composed of minerals like limestone, dolomite, aragonite, coral, shells, etc. But it rarely will move the opposite direction, softer, at least not to any great extent.
The KH is usually close to the GH, and we can ignore it for the moment. Which brings us to the pH. The pH of the source water is governed by the GH, KH, dissolved CO2, and possible factors in the water source or sometimes substances added by the water authority. You always out-gas any CO2 in tap water before testing in order to get a more accurate result. The pH can alter in an aquarium, up or down, but it depends upon the level of the GH and KH. The higher these are, the less fluctuation the pH will have. This is why adjusting or attempting to adjust just the pH in an aquarium is so risky and downright dangerous if fish are present. The GH and KH must be dealt with as well, especially when softening/lowering pH. Increasing pH involvees increasing GH and KH. This is a somewhat complex process, and really should never be done inside the aquarium, so water has to be "prepared" outside. We can leave this for the present.
The substrate is unlikely to affect GH/KH unbless it contains calcareous substances. It is unlikely to lower the GH. The pH might be impacted, and the extent of this will be determined by the initial GH and KH.
Colin has dealt with the water GH, it is soft. It is not high enough (in GH) for livebearers or rift lake cichlids. So look at basically soft water species...most of the commonly available tetras, rasboras, danios, barbs, catfish, loaches will be fine.