It's a tangled mess, but it doesn't need to be.
The best advice I can give to anyone starting out injecting C02 is this.
Use 3 methods of calculating co2 levels. Ph change, drop checker colour, and the most important measure by far... how are the fish reacting.
Ph meters are accurate enough for our needs, the ph change is the important bit. Whether or not it was lab grade calibrated is imo, a distraction.
Monitor the ph and look at the dc, make photographic notes how the dc changes in relation to the ph. Obviously understand the dc lags by 1 ~2 hrs.
By doing this you will quickly learn to guesstimate the tank ph based on the dc colour, or guesstimate the dc colour based on ph meter readings.
I've just eyballed both dc' in my tank, I guessed one was sitting at 6ph, the other at 5.5. I wasn’t that far out. 😆
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