I'm happy I stumbled on this thread.
I have one tank (out of three) that always has me nervous as kh is always less than 1, but the fish are doing better in this tank than any of my others.
I used RO water and equilibrium brought up to 6gh with ecocomplete substrate with a ph that's around 6.6-6.8.
I've been nervous about the reported "ph crashes" but the success of the bolivian rams, wild caught neon tetras and otocinclus have prevented me from changing any of my routine up.
My other two tanks must have substrate that's leaching something in the water as my ph on both of those tanks is 7.6-7.9 with a kh of 4+. My fish don't thrive nearly as well as those tanks and the plants do about the same in all of the tanks. It had been puzzling me with a slight bit of worry about the PH crashes, but after reading this I'm going to redo my other two tanks so that I can get more similar parameters across them all.
A buffering substrate like aquasoil is interesting, but it sounds like you would need to replace the substrate every so often (every 1-2 years) if you notice your PH starting to raise and that doesn't sound fun to me.
I'm pretty new to this stuff (only one year in) so am happy to read any corrections to my assumptions.