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?

 

I'm always gona worry cause I don't understand But essentially all I am trying to do is rule out co2 as an issue or make it non limiting.
I do run surface agitation and a skimmer and the co2 is off 2 hours before lights out.

Have noticed a change over last week though after I did a massive trim of plants, so you might be on to something there.

About 6 months.
It's well maintained. To the point at least that I am trying to maintain it well if that means anything.

I noticed a small amount of diatoms today, so maybe the bio filtration stalled and struggled at 5.5 ph but also trimmed and replanted nearly everything so will give it some time to stabilize and see if it continues.

 

Is there a difference between the two? I don't have an airstone. But can run a little pump in a bowl probably

Plants are pearling well, towards end of cycle everything is covered in large bubbles.

Just not heard of a tank gassing off so little co2, I thought maybe there was something else at play.