After you’ve run all the tests again, if the pH of your tap water (and your tank) is confirmed as 8 as you mentioned earlier, and the pH of the fish store is much lower, say 6.5, then I wouldn’t place the fish from the store in my tank immediately as this would mean an immediate change of pH by 1.5 which will affect the fish.
What I’d do in that case is to lower the pH in my quarantine tank in preparation for the fish so that my tank’s pH is equal to, or no more than 0.5 unit higher than, the shop’s pH (if the shop’s pH is 6.5 I’d ensure that the tank pH is no higher than 7 by mixing my tap water with RO or rain water, before placing the fish in the QT). Every 24 hours I’d add a bit more of my tap water to the QT to bring the pH up a bit more until the pH eventually reaches 8 which is the same as the tap water’s pH.
After that, water changes will be done using tap water, and hopefully there’ll be no loss of fish.