I have a 90P tank witih an Eheim skimmer, Oase Biomaster 600 thermo and CO2 art inline Pro diffuser. I've managed a steady and high concentration of CO2 that drops PH by 1,02 throughout the photo period. CO2 goes on 2,5 hours before lights start ramping up (Twinstar 900S, ramping 30 mins from 0 to 80%). The CO2 art built-in bubble counter produces such small bubbles it's impossible to count them using ones eyes only.
We have quite similar setups (hence why I'm sharing the info above on a tank that runs very well for me), I'd suggest you increase the CO2.