I would try again with a much longer acclimation 1.5-2 hour drip. Use a bigger bucket. Ask the LFS
what their readings are as well PH
temp and salinity, then you will have a better idea how much change there is.
What are your long term stocking plans? Maybe something other than a tang to start with. I had the exact same problem early on with tangs. Now I have 2 with no problem. My tank was over 6 months old before I was successful though.
would also be a good option although I realize not all abide by this myself included when I first started.