If your car isn't overheating stop changing the oil.
Water changes keep things right. I am skeptical of 0 water changes. Even when it 'works' most likely the tank would have done even better with water changes. The tank may still do well with reduced size or even frequency, but I would never stop no matter how good the numbers are.
It isn't just nitrate that builds up and harms the fish. There are growth inhibiting hormones and dissolved organic compounds that we don't have test kits for. These generally correlate with nitrate unless something is removing nitrate specifically (live plants, certain filter medias, etc.).