What test kit are you using? If you have API kit for nitrates, please get a different brand. Apparently, part of the ingredients settle and harden in the bottle if you don’t shake it up every few days. It will either give you a zero or a very high number. Mine read zero for a long time until I got a Salifert. My nitrates are really more like 10ppm and not zero.zero nitrates for about 6 months