Wow, your tap water's pH is through the roof--you could use that for marine aquaria! Is your tap water what you're using in your semi-hydro?
I think you might be overly-generalizing about what pH is suitable for S/H growing--that's going to depend hugely on the type of plant.
Don't do a drastic pH shift--gradual changes down would be much better. Here, I buffer our fert solution to 7.0 and have very good results.....
What genus of seedlings will you be conducting the tests on?