You could have huge honking dirty goldfish and the nitrogen from your fish tank water would not be too high. It's just a tiny bit richer than plain water and you know it doesn't have chlorine in it anymore. (Nothing against huge honking dirty goldfish! I speak from fond experience.)
They were talking about Fish Emulsion, a goopy liquid chocolate brown stuff from a bottle that smells like chum. You mix it in water to fertilize plants.
Oh, I just started to siphon my fish tanks, pinched it off and then ran the hose or tubing out onto the lawn or into deep buckets. With large comets, I'd do a 50-75% water change every week in a 100 gal. tank. Ymmv, but that mulm accumulated amazingly fast with healthy fish and a varied diet, in a planted tank with two types of filtration. You'd have to be more cautious with water from hospital tanks, or salt water set ups, and you can't drain aquarium water off into local bodies of water because of potential aquatic invasives and pathogens.