Of course you can top off the tank. It is just an inevitable fact that without plants, you will eventually see ammonia/nitrite/nitrate. Now if the tank is cycled, there won't be a nitrate 'spike' just a reading, eventually you will get a clear 5ppm or 10ppm reading. In a cycled tank, you do water changes to dilute waste products (nitrate is just the one we test for as an indicator). If you want to verify your ammonia test kit(s), fill a bucket with water, put 1 drop of ammonia in there, stir and test.