Unless you are on a private well in a rural area, your tap water nitrates should be under 10ppm. The MCL for all US public water suppliers is 10ppm, and no legitimate public water supplier would risk going over the MCL, unless they have an ideology like Flint.
Because you have not responded with your water change schedule, I will add mine to the fray. For me to maintain a nitrate level between a 2 and 5ppm, I found I needed to change about 30% of my tanks water every other day
Because you have not responded with your water change schedule, I will add mine to the fray. For me to maintain a nitrate level between a 2 and 5ppm, I found I needed to change about 30% of my tanks water every other day