What's your tap KH?
Is your substrate EcoComplete by chance? I have very soft tap water, but Eco raised KH in my 29G from undetectable to 6. In my case, water changes would gradually bring KH down (like what you're seeing) were it not for the hardscape dissolving due to acid from CO2 injection.
Obviously baking soda is your friend, but I wouldn't start messing with water chemistry until you figure out the steady state, especially if your CO2 is automated. If your KH is being driven up or down, pH will follow and automated systems will attempt to compensate. This can kill fish at extremes.
Example (from my very own tank):
Hardscape dissolves, raising KH
pH rises with KH
(If my CO2 was automated) System injects more CO2 to lower pH
Increased acid causes KH to rise
pH follows, and system again tries to compensate by injecting more CO2
You see where this is going...