Sorry to hear.
It almost positively was not an ammonia issue. If you have corals in the tank and they are doing well, it is virtually impossible to have an ammonia issue unless something went really wrong, like having something large decompose in the tank, or adding chemicals that kill the bacteria. Nitrite is not toxic to marine fish.
I think that we ruled out an acclimation issue, but there still seems to have been an immediate change in the fish going from the store to your tank. One thing that can happen - fish get struck by the net frame when being caught at the store, and then the fish is put in the bag, and you can't tell it was injured until you get home.
Jay