I have made a few changes since I last posted, hopefully most of them for the better! First I have a new filter, an Aquael Ultramax 2000. I feel this is much better suited to the tank size than my old 306. I also purchased an inline diffuser, which appears to be doing a good job.
Interesting pH profile this week. The pH dropped slower and not as far as previous weeks.
Just giving my thoughts on your CO2 Injection. The drop being slower in PH could be due to your filter losing flow from the filter pads being a little bit dirtier overtime. IMO it's better to have injection from another pump and not using the filter for this.
I experienced a bit about CO2 injection in the last few months, you can improve your CO2 injection efficiency by augmenting the turn over rate of the water where the CO2 is injected (Works best for inline injection). I guess with an in tank diffuser, the overall turn over inside the tank should help aswell. (Did not experienced much with in tank diffuser, just a thought).