Oh boy, what a question. There's a lot to say about water in general, as well as about tap. If coming from a municipal supplier (as opposed to a well on-site) then it's required by the EPA in the US that it be treated. The EPA is encouraging munis to use chloramine instead of chlorine because of cancer issues. Chloramine is also much more stable. Munis may artificially harden the water to preserve piping, and that leads to high alkalinity (alkalinity not as a descriptor of pH, but of buffering capacity). However, their source water may very well be of a high alkalinity in the first place, needing no help from the muni. It is that mineral composition that can play a huge, huge role in all sorts of problems. The only way to know for certain is to test the source water.
For instance, I know that my tap (well) water has a general hardness of 10, which is reduced to a GH of 6 after boiling for several minutes, and a dKH of 7-9 (German hardness), which is reduced to 4 dkH after boiling. Because I can reduce total levels by both measures of hardness, I can state unequivocally that my water has temporary hardness. In a pinch I can boil it, but I'd better not need dozens or even hundreds of gallons.
Reverse osmosis, or reverse osmosis with deonization, removes most, if not all sources of hardness, thusly eliminating all alkalinity (buffering capacity, or resistance to pH shift) of water. It also strips it of O2 from some of what I've read, which leads to lower pH until the water reaches a CO2/O2 saturation equilibrium.
I'd be curious to know what your source water's pH is along with that ppm level, along with hardness points.