As long as you have high nitrates your pH is likely going to be low. The acidifying effects of dissolved organics lowers the pH. Do some water changes, and as that will lower your nitrates and the associated nitrites, and ammonia, it will raise you pH. If the pH was associated with calcium problems of your salt mix or acute problems with your source water the pH would have been low before the nitrate readings went up. Salt water mixes only contain so much in the way of buffers, and your nitrates are obviously overpowering the buffers in your water. If your circulation from your prism any other filter is adequate you should be having adequate gas exchange to shed any Carbon dioxide. How low a pH does your pH color indicator capable of reading? The easy way to tell if its at the end of the cycle is to do a couple of water changes, test for nitrates, add a little fish food and test 1 and two days later for nitrates. If the cycle is over the nitrate reading should not go up appreciably.
