In my opinion your pH is not that high, 8.4 is the ideal level, thus give it a plus/minus of 0.2, and you get 8.2-8.6.
Also note that once your tank is running, the chances are that it will go down on it's own. My tanks are running at 7.4, but they are fish only tanks, no corals.
As far as your nitrates are concern, you will need to bring them to no higher then 5, for corals but ideally at 0 (zero). Thus the big issue in
saltwater fish/coral keeping; "How to bring the nitrates down?".
This is a very compiated process, if not, at least varying in methods. The most commond method is water changes. Skimmers are used to help reduce the proteins before they can become nitrates. There are other methods aswell, remote deep sand bed, macro lagae or turf filters are some. Personally I do not keep corals, thus I really don't worry about it too much. The mothod that sounds the most convincing to me is the turf filter.