I have some rather high nitrates in my tap water at certain times of the year. If your tap is around the 40 mark, it may be that you will not notice any change. When you get your nitrites and ammonia under control, we can start trying to work on the nitrates. In the meantime, your ammonia and nitrite changes will be quite enough to deal with any excess nitrates so I wouldn't even test them except to practice with the kit.