Kind of... pH does definitely have an effect on fish well being on its own. Try keeping fish in pH 2 water! or pH 12. Dissolved CO2 reduces pH by the formation of carbonic acid which is a proton donor (weak acid, puts H+ into the water) and pH is a measure of H+. The "p" in pH is an operation that means "take the negative log" of H which the concentration of H+ (not H-) ion in molar units. pH levels and mineral content are not entirely separable. The dissolution of different metals in water changes pH. If you drastically change pH you will precipitate out some metal species and others will come into solution.
Anyways to OP- why are you opposed to using RO water?
The Ph itself does not tell you the amount of hydrogen ions in the water. So at s ph of 2 one tank might