It depends on what fish you want or have. Also, if you have cycled the tank already. Ideally, after the tank is cycled, the ammonia should be 0, nitrites should be 0, nitrates should be between 0-40, 0-25 for more sensitive fish, P.h. should be between 6-7.8 (6-7 for fish like tetras, 7-8 for some cichlids) with closest to 7 being best.