Two main causes for red streaking in fish is either bacterial or from dramatic drops in pH. In either case, large daily water changes with well aged SW
are the best "cure". If bacterial, it will dilute the DOC
that feeds it and if chemical it should bring things back into balance. Be sure the tang is getting vitamin fortified foods as well.
If the parameters do not come back into line after a few days, I would remove the rock and cure it seperately. It will just prolong the problem with the tang until it becomes severe. The ammonia present in the water is most likely what's affecting the inverts. Most once acclimated to a stable environment cannot handle the shock. That coupled with the hypersalinity may have put them over the edge.
Next time trust your instincts.