Fishless cycling?

In my opinion, you can keep adding ammonia when it is close to 0ppm. Anywhere around 0.5ppm could be considered as near zero.

What test kit are you using? API Freshwater master kit is sometimes tricky to use. The nitrate test requires extensively shaking of the bottle before using, and more shaking is needed after you have mixed the two liquids up. Otherwise you might get a false reading.