Its so unfortunate that so many are having difficulties with beatiful GBR.
GBR which i am famiiar with are shipped in very small bag with ver small amt of water from SE Asia (not even shot glass worth) but still managed to survive and breed when properly acclimated/handled.
All the fish are shipped in slightly acidic water than should be to minimize the toxiciy of
NH3 produced during transit including s/w fish.
If they land in NYC, not a problem but if they are unpack and acclimated in an area with hard water (high pH), probably higher mortality rate. Yes it can adpat to higher pH than specified but should be acclimated to same pH as unpacked water then may alter the pH gradually after they are well adjusted in new tank conditions. I am talikng not days but weeks or months in order to reduce their mortality rates.
Your lfs maybe keeping in 7.5 which doesnt necessarily mean they will last long time since they are there to sell, not hold the fish. So you will never know the exactly how many will surive and for how long atbhte lfs.
LfS by me always have tankful of GBR (50 or more) where I can Pick out many breeding/mated pairs everytime time I visit.
Do you have any way you can soften the tankful of water (Inert material container to hold and circulate and maybe a heater)?
You can use Peat Moss in nylon bags in the container and softened the water which will drop the pH of the what now is aged tap. You may want to add pc of true
driftwood (no slate attached to wood) in the tank. Should boil the DW somewhat to clean. These process can tint the water a bit which can be dealt with at later time after fish have adjusted to your tank and doing well..
Another important thing is to find out the regular arrival date of GBR (Mon, tu, etc). When possible (Only if they get in many, 50+), you can wait to see how new arrivals are doing before actually buying. this means you may have to visit store on daily bases just to see how well they are doing.
I know, this maybe too much for one fish but I used to do this to learn more and find the way to acquire healthier specimen (higher specimen).
Do you happened to have extra tank w/ heater and simple sponge filter?
Pardon me for more questions but trying to understand the situation better. Is your tank ph as high as tap as posted?