A Question On The Co2 Ph Drop

Short summary is that my rested PH from tank after 24 hours reads 7.10 PH.
But my tank before Co2 turns on reads 6.15 PH, both measured digitally and fluctuations over the months are less than 0.10 Ph.

Fish gasp at about 5.45 with light surface movement and skimmer.
Current Ph sits at around 5.5 - 5.55 at its lowest and 16 hours later when co2 turns on again its barely increased by .5

Is this just excess Co2 that the system can't off gas. It seems like a lot and I am wondering of there is some underlying cause or
Other things playing a role in the water column.?

I have really struggled with Co2 on this new tank which has helped me learn a heap but I inject it to the point where I can't inject any more and still have plants melting and other low co2 issues, so can't yet rule out having good co2.

Any thoughts?