Why can't i bring my pH up?

[MTDCTL]

I've been in the hobby for 4 years and i still cannot figure out the chemistry behind maintaining a stable pH. My parameters are:
Mg- 1300ppm
dKH- 12.6
Ca- 440ppm
pH- 7.4
Salinity- 1.024
Phosphate- 0.02
Nitrates-0
Ammonia-0
Nitrite-0

Can someone tell me what i may be doing wrong? Thank you

 

[Randy Holmes-Farley Verified Community Expert]

I think the measurement of pH 7.4 is not correct, but in general, pH is determined ONLY by the alkalinity and the carbon dioxide level.

With normal or high alk (as you have), you need very high CO2 levels to attain pH 7.4 (getting close to unbreathable in the room, since tank CO2 is often driven by room Co2 levels)) and the rock and sand would be slowly dissolving, at least if the alk drops closer to normal seawater levels of 7 dKH.