well I use Nestle pure life water because it has a ph of 7.3 witch I herd is a good amount for cannabis plants , but I will cut off their feeding UNTILL necessary I usually feed them a half teaspoon in a gallon every 4th or 3rd wateringWith that soil and that humidity, I would just use straight water with a ppm of 100-150. Using harder tap water you could dilute it down to those numbers with RO. Or using RO you could add Calcium Carbonate and/or tap water to bring it up to 100-150ppm.
When it comes time to start adding nutrients start really low, and I personally wouldnt go above 500ppm, on a feed/feed/water cycle with humidity that low. Probably wont be until they are in flower or really close to it. But obviously you will need to feed them more specifically based on how hungry or overfed they look