To make a quick summary of it, having good water quality is not only down to the amounts of nitrogenous compounds in the water column. Water quality is also affected by the toxins released by biomass that accumulates due to uneaten fish food, fish poop etc.
Rather than just applying the generic name of "toxins", is there any literature documenting what these toxins are?
I find the easiest way for me to understand something is to try and pick holes in the theory behind it. I struggle to do any task if the rationale is "because someone said so"