Knowing when ferts are needed
You will know that additional ferts are required when crypts, swords, and rooted plants have a lot of roots emerging from the gravel. Plants will look pale. You may see long green algae -- I remember when I had a fancy goldfish that hated nitrates and the long green hair algae got so bad it twisted into dreadlocks!
I like the root suppliments, I think I have AP Root Tabs plus I have a "roll your own" kit from Plantguild to make cuctom root ferts.
Member AGA, HAS, NASH
110G, 10 adult discus, barebottom with plants on wood
58G, 3 adult discus, barebottom with plants on wood
59G thickly planted tanks, rummey nose tetras, kuhlies
300G goldfish pond outside