Hey Nycreef,
Sorry about your Tang.
Stress factors are intangibles and it is hard to say if changing the aquascape or even the presence of additional people might have affected the fish. I doubt that these changes alone would have caused the death.
I do suspect that addtion of a 20 lb. rock might have affected the water quality or even caused an Ammonia spike. Did you "recure" the rock in a separate container before adding it to your tank?
Even "fully cured" rock can experience some die off due to shipping, transporting or the different water conditions in your tank.
Again, sorry about your loss.
Dick
