Substrate fertilizer tablets are fertilizer, so you are using fertilizer. They may contain nitrogen. Read the label. Do not add more until the nitrate issue is resolved.
Be very careful not to disturb the substrate, the fertilizer tablets might be too close to the surface, or even exposed somewhere. This could very well be the source of the nitrate.
Sorry to threadjack, but I just read this and realized that I made this mistake. I think I put in too many root tabs, and some are quite close to the substrate surface (enough to see crumbled fertilizer debris exposed where the substrate was disturbed).
My tank is still cycling so my nitrates are 40-80ppm. Not sure how much is attributed to cycling and how much to my root tabs (which do contain nitrogen).
Will this affect my cycle and what do you recommend to do?