Anything decaying will produce ammonia. If there's a dead leaf, or something in the gravel, etc, it will produce ammonia.
the other possibility--how high does the ammonia test read? If it's maxed at 4, that means you could have ammonia of 8 ppm, and the test kit still will just read 4.
the other possibility--how high does the ammonia test read? If it's maxed at 4, that means you could have ammonia of 8 ppm, and the test kit still will just read 4.