If you're doing aggressive enough EI then, in theory, you don't need to monitor.
In practice, I monitor NO3 with the Salifert test, PO4 with the API test, and GH occasionally with the API test. I've confirmed that the Seachem iron test works only for free iron or Fe gluconate; it will not detect Fe EDTA or Fe DTDP.
I'm pretty sure something is being used up as fast as I can add it to the tank, but I don't know what yet. I may take Surf's advice and have the water tested.
In practice, I monitor NO3 with the Salifert test, PO4 with the API test, and GH occasionally with the API test. I've confirmed that the Seachem iron test works only for free iron or Fe gluconate; it will not detect Fe EDTA or Fe DTDP.
I'm pretty sure something is being used up as fast as I can add it to the tank, but I don't know what yet. I may take Surf's advice and have the water tested.