I think your tank water contains not only dissolved organic compounds extraneous to the tap water but also inorganic, so a simple reference to the tap water might not mean much.
Moreover, the mobility of large organic ions through the water can be orders of magnitude different from small inorganic ions, which make up the current which is being measured by the TDS. It is not a straightforward analysis, AFAIK. I mean a small organic molecule like an anion of the muriatic acid can move rather quickly, but a large one like an anion of a fatty acid will move slowly and even affect overall dielectric properties of surrounding water because of its very large and long hydrophobic molecular structure.
Besides, it appears we are mixing two unmixable things - dissolved chemicals and suspended microscopic particles which are not dissolved and cannot be registered by a TDS meter because they bear no effect on water conductivity.
Clearly you know a lot more about this than I, so much so than a lot of this goes over my head. So the organics I mention are dissolved chemicals, and the organics he mentioned are organics suspended in the water column as particulate to cloud the water? I just want to make sure I understand it correctly. That makes these heavily stocked tanks a lot more scary.