Free ammonia is what's dangerous. The higher the pH, the more free ammonia is present (by percentage). This chart explains that.
CNYKOI - Ammonia calculator
By this you'll see that, at 0.25ppm, the difference in free ammonia at 7.0pH and 8.0pH is not harmful at tropical water temperatures.
Of course, once your tank is cycled, this is a moot point.
EDIT: I don't see Prime affecting pH either way. What water conditioner are you using that does?