CO2 injected tank and quarantine tank

I've decided to inject CO2 into my 75 UK gallon (330 l) tank. This is my
main tank and usually has a pH of 8.1. My quarantine tank stays around a
pH of 8.5.

I'm adjusting the CO2 bubble rate very slowly, in the main tank, to get
it down to about pH 7.4. Now a problem has suddenly arisen because of
the large pH difference between these two tanks. I can transfer a fish
from the main tank to the quarantine by placing it into an intermediate
3 gallon tank with aquarium water and waiting for the CO2 to dissipate.
But how do I get a fish from the quarantine tank to the CO2 injected one
without killing it with the pH difference?

The only thing I can think of is to start injecting the quarantine tank too.

What's the maximum pH swing per day a fish can tolerate?

Nikki