Alkalinity keeps going low

[TLWR]

~250 gal 3 person tub, 2 months old.

Nature 2, Ozone (24/7 via circ pump), 1TBS MPS upon daily use, 1TBS Dichlor weekly.

I have noticed that my alkalinity keeps going low and PH goes low by a small amount with it.

I add Baking Soda to bring it back to proper levels, at about 4TBS per week. That seems excessive to me, I thought once I got the Alkalinity set initially, it would not need to be messed with much.

This is city water (have not tested fresh tap water) what would cause my Alkalinity to keep dropping, and is that normal?

[chem geek]

Both MPS and Dichlor are net acidic after accounting for their usage/consumption so in that situation you will most definitely have TA drop over time (it's just like adding acid to your spa) and need to add either baking soda or pH Up to increase it back to where it was.  The only thing that a certain TA level will get you in this situation is relative pH stability.  That is, at a certain TA level, the rate of pH rise from carbon dioxide outgassing can equal the rate of pH drop from the net acidic sources of chlorine or oxidizer.

7 tablespoons of MPS and 1 tablespoon of Dichlor (i.e. one week of chemical use) would require 2.6 tablespoons of baking soda to compensate for the drop in TA, assuming that carbon dioxide outgassing kept the pH constant.  You are needing to add more because there isn't enough carbon dioxide outgassing so the pH is dropping somewhat.  At a higher TA level, you can get that balance in pH, but you still need to add something to restore the TA (carbonates that are lost as carbon dioxide).

It's when you use a hypochlorite source of chlorine that the TA is fairly stable and where a lower TA to minimize carbon dioxide outgassing has the pH be more stable.

[TLWR]

Thanks for the detailed and understandable reply, that makes perfect sense.