coral raises ph but its expensive to do a coral rock decor, you could always do a crushed coral substrate. Also you could do african cichlid live sand like this
http://www.amazon.com/Carib-Sea-Ecco-Complete-Freshwater-African/dp/B0002DH0X0 (<-thats what I use as well as tanganyika buffer) but on the top of your list of things to get should definitely be the necessary test kits, ph, ammonia, nitrites, nitrates. without those you dont know whats really going on in your tank, and especially with tangs, they are pretty sensitive fish.