I agree with rajin_cajun, although nitrates are the basic indicator of a cycled system, they are hardly the end all of water changes. Fish are constantly producing hormones and pheromones, some of which may be growth inhibitors to other males or other species.
And when we remove old water and replace it, we can be adding buffers and other components that aid the health of the system.
And to the buttikoferi, if there's a fish that will tolerate a trace of ammonia, buttis can, they are known to follow hippos in rivers in Africa and eat the feces as they spew from the hippos arse.