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pH, CO2, and snail health in a planted tank

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Hi everyone!

I had corona nerite snails in my tank, but they eventually all got pitted shells and died. My tank is planted with CO2 injection. I have very soft tap water (2 deg KH, 1 deg GH), so I raise the KH to 3 by adding baking soda with every water change. My pH without CO2 is 7.6-7.8 (it fluctuates throughout the day and week, not sure why, maybe plant respiration plus increasing nitrate levels?). I inject CO2 to bring it down to 6.6, shooting for 20-30ppm of CO2.

Why do the snails have pitted shells? Is it because the pH is below 7? Or is it because the GH is too low? Another way to put it: if my GH is higher, can the pH still be 6.6 without causing the snails' shells to break down?

I also have a brood of MTS, but they never grow beyond 1cm in length. The original parents were 3cm. The MTS also have weak shells.

Thanks for any advice,