i think it was when i doubled up on my KNO3 dry dose, to initially raise levels. i originally had <10ppm. I've done major waterchanges since, but i think a lot of it remains in my substrate. i also have root ferts planted in the eco-complete.
i stopped dosing ferts about 2.5 weeks ago in hope the poants would do the rest... but then again my plants aren't the fast growing variety. i was thinking if adding some pennywort from my fry tank for added plant mass and oxygen boost, but not as a nitrate sponge as the fish seem fine