Good Chemistry-loosing new fish-confused

Hi,
I am truely hoping someone can shead some light on why I am loosing new additions to my tank.
I have a 55 gal with 2 powerheads, a Biowheel filter and a protein skimmer.
It's been up and stable for 3 years.
In it I have approx 45 lbs live rock, live sand, one Chromis, one Percula Clown, 3 Turbo snails and a Yellow Tang.
Water parameters
are:
Ph 8.2
NO2 0
NH4 0
NO3 10-15ppm
Temp 72
Salinity 1.021

I had the clown, chromis and a blue fin damsel in the talk since the begining.
Because the Damsel was a bully I did not add any new fish. He finally passed away a couple months ago. I inherited him from a guy who was breaking down his tank so he was an old fish.
3 weeks ago I went to the LFS and bought 2 Anthias. Having never owned them I asked if they were easy keepers and would fit into my tank with the clown and chromis. Was told they were and would. Well they were all dead the next morning.
I acclimated all my fish via drip method.
LFS gave me a credit in which I purchased a Yellow Tang and Strawberry Pseudo.
Strawberry was dead the next morning. Yellow Tang is doing great.
Again all parameters were as above.
I then waited a week and purchased 3 Anthias from another source. Same as above, dead next morning. Tang still good. Parameters still good.
Yesterday I purchased a Coral Beauty from yet another LFS. Acclimated, checked parameters etc. Was dead this morning.
Each time I have questioned why this was occuring I have been told it's bad water chemistry except for the Anthias who from what I've been told aren't very hardy.
Yet the Pseudo and the Coral Beauty are suppose to be. And I've had a Coral Beauty in the past in a smaller tank with no isues.
I now am afraid to add anything else but frustrated at the same time as there is nothing that I know of that would be costing me the loss of the fish except for the Tang.