I just tested for nitrates and as usual ask my girl to tell me the results as I am color blind. Anyway, I have never experienced this before and I have maintained numerous tanks. Last night trates were 20ppm. I did a 5gal w/c on a total volume of 18gal. I waited two hours, retested, and asked her once again for the results. She says they are still 20. There is no 15ppm; the kit goes from 10 to 20. Logically, it makes absolutely no sense to me. I guess I will have to do another water change tomarrow.