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Hard, Alkaline water

Our community sits on top of a slab of dolomite. Water hardness is 
about 300ppm  and the alkalinity, about 300ppm with a reading of 
about pH 8.3. I've been cutting with RO water to achieve KH 9. Adding 
CO2 results in lowering the pH, and I can use the charts to determine 
the CO2 content in the water, the target being around 15mg/L.

Question: If I use straight tap water, add CO2 and measure the water 
via Hach titration method to identify the goal point of CO2 15mg/L - 
30mg/L, I may still have a rather high pH. Am guessing here.  In this 
situation, is high pH a problem for tropical aquarium plants?

A colleague mentioned, the possibility of changes in the mobility or 
availability of various ions starts, as pH changes. "For example, at 
very low and (I believe) at very high pH, P may start precipitating - 
with Fe under acid conditions, with CaCO3 under alkaline conditions. 
And the invisible but extremely important nitrifying and denitrifying 
bacteria may be affected."

John Glaeser