yes my tap is practically zero i have super filtered water here probably why it costs 450 dollars every 3 months lol, i guess i would have to take my water in and let someone who can tell the different colors do it cause i cantNo, C02 isn't needed for the slow growth plants you have. I was just saying that the fast levels of growth needed to have plants really reduce nitogren compounds required C02.
Have you verified that your source water isn't introducing nitrates?.
also looking into duckweed as its great t reducing nitrates