This was my experience a few months ago... until I was told that I had to shake the test bottles until my arms were sore. Something about crystals having to break up before it could test properly. Now, if you're having 0 nitrates, it could be 1. The cycle isn't complete and you're still going from ammonia to nitrite, but the bacteria colony for nitrite to nitrate isn't sufficient yet. 2. The test may not be accurate (not shaken enough or potentially expired tests?) 3. If doing huge water changes and water is not dechlorinated, it may be causing the bacteria in the filter and tank to die, hence restarting or delaying the cycle. Personally I use water straight from the tap when doing water changes but I only do 15%-20% at the very most so it's usually diluted enough that the bacteria aren't too adversely affected.