Then the issue is your water.....not the soil. I don't know your hydro guy but mine is excellent and both he and Stinky recommend Happy Frog for seedlings. Find yourself a better source of info imo. The only thing I've EVER put added to any of my soils is perlite and have never had any ph issues at all.

Consider RO or distilled water. We have well water where we live and it doesn't matter WHAT you use OR how often.....the ph will NOT remain stable. Every ph issue we had (and we had TONS.....including with our tropical fish) immediately stopped upon switching to RO water.