All the organics generated by the fish and bacteria are wastes (they would not be excreted if they were not, they would be conserved), plus excess phosphates, sulfates, chlorides, oxides, etc. from foods and evaporation concentration, hormones and pheromones, cyclic organic such as phenols (half-life in aquatic systems ~18 months, and they stink!). The tiny handful for things which we can measure are trivial compared to the myriad we cannot test. Nitrate is an indicator. It is toxic itself long term, but the levels for toxicity vary hugely with the type of fish. It serves better as an easily tested indicator of general pollution. Remove that specifically and particularly and all you have done is blind yourself to the organics. As with a great many things about fish tanks, folks get an idea and never really test it against the real world. Water changes are cheap and easy and out-perform all the absorbtion resins and devices in the world by a very, very wide margin.