You probably don't need the stress zyme. Once a tank is cycled, why add bacteria?
Why are you adding anything to the water for pH? Is it in reaction to this drop at water changes, or is it what is causing it? Is your tapwater slightly acidic too?
In response to PNW ... I've spent some of this evening reading up and I'm still not sure I believe it's useful to consider baking soda a base in this setting.
What I'm gathering, and I could be wrong but I think I'm on the right track ...
Baking soda in the box is a salt. It can react with both acids and bases.
Combined with distilled water with a pH of about 7.5, the result is slightly more basic. That is, the solution of baking soda and pure water is basic ... This doesn't mean baking soda is a base.
In water with a pH of about 8, baking soda has some efficacy as a buffer, to keep pH where it is. But not a lot.
If baking soda were a base, surely it would continue to raise pH the more you add? Since it is preferential to a pH of 8, sounds like a buffer?
So I still think at this moment it's easier to understand baking soda as a buffer in aquariums, albeit a weak one.
The OP might look into DIY Chiclid salts. I believe these combine baking soda, Epsom salt, and ice melt product, to raise KH and GH. The byproduct of which might be a more stable pH.
Sent from my iPhone with three hands tied behind my back.