i'm throwing out a couple of scenarios:
1. doing a "decent amount of substrate disturbance..." may be what's causing your problems, particularly if you do so while draining 50-70% of your water. agitating the substrate then draining could still be leaving excess nutrients in the water column, feeding your bba. you don't need to vacuum substrate, especially aquasoil. try being less aggressive in your water changes--you really only need to siphon out water just above the substrate level. spot treat with excel then modify water change method.
2. your pH probe needs to be calibrated and that the pH levels (hence co2) are not what you thought.
3. try to keep things stable as possible. you've been chasing it for a year now.
i think #1 is most likely.
1. doing a "decent amount of substrate disturbance..." may be what's causing your problems, particularly if you do so while draining 50-70% of your water. agitating the substrate then draining could still be leaving excess nutrients in the water column, feeding your bba. you don't need to vacuum substrate, especially aquasoil. try being less aggressive in your water changes--you really only need to siphon out water just above the substrate level. spot treat with excel then modify water change method.
2. your pH probe needs to be calibrated and that the pH levels (hence co2) are not what you thought.
3. try to keep things stable as possible. you've been chasing it for a year now.
i think #1 is most likely.