Many factors can affect the pH. For a new system, I would suggest keeping the pH JUST UNDER 7.0 because this will prevent unionized ammonia in the water (which is what kills fish). This will make the bacteria take a little longer to reproduce, but the plants will pick up some of the slack by absorbing some of the ammonia directly anyway. Shoot for a pH of 6.8 to 7.0. Alternatively, you can just let the system run wild and "Darwinize" the fish. The hardier fish will survive, and you can replace the weaker ones at 20 cents a fish... The remaining fish should handle the extremes of an experimental AP system much better