I do waterchanges on cycling rock when ammonia goes over 1 ppm or nitrate goes over 5 ppm. Sometimes I'm fussier than that. Ammonia will kill off life hitching a ride on the rock, and nitrate will get absorbed into the rock (later releasing from the rock and causing nutrient issues like algae). Unlike freshwater, nitrite is non-toxic to marine life until very high range (think ~100ppm) so don't worry about nitrite.
Check the link in my signature for lots of info on cycling.
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~ Mindy
SPS fanatic.
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