ammonia = 0
nitrite = 0
nitrate = 0 if possible, but levels up to 20 ppm are generally tolerable
ph = 8.1 to 8.3 being the ideal range. 8.0 and 8.4 are right at the border of being too low or too high.
It depends on the specifics of the situation. What is high and why. The 'why' part requires some though and experience. If you tell us more about your tank we can probably point you in the right direction.
