I do not use ferts... I stopped using Flourish Comp... I do use root tabs...
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.