But once bag opened co2 is gone, new fresh air is in, so even if we close bagels again ph will climb due to the air exchange that occurred when u opened the bag..no?You probably don't ever have to deal with that unless you're getting the fish shipped, and it's been a long time in transit. This is more a concern on the collector to wholesaler side where the fish may really spend a loooooong time in transit.
But, you would open it up and immediately use a digital pH meter. (You could then close the bag again air tight). It isn't instant, but it's fast - minutes. It's difficult to get small adjustments in a bucket on oG so I usually use a 20 to 30g tub when I need to do this. You would want to get the temperature close also. I would probably use a tablespoon of acid in a cup or two of saltwater, and then out of that use drips of it to go down to the target. Flood with methylene blue too, increases hemoglobin's ability to bind oxygen and also chills the fish out a lot. When 20 to 30min are up, then drip to bring the pH back up to the destination (I like to do 7.6 to start so the delta is less) and when pH and temperature is matched, you can net the fish, then move it to the destination. If it's doing okay I'd probably do a freshwater dip in between, too.