Bacteria that import nitrate, phosphate (or other types of negatively charged ions) also need to import a positively charged ion to maintain ion balance. The cation that is used the most for this role is potassium. Sodium is more abundant but it causes protein precipitation in high amounts, so it is not used extensively for this process.
NO3- and PO4- import is either conducted by a K+ symport or an other membrane pump independently pumps in K+. Most common pump for this would be a H+/K+ antiporter that pumps H+ out and K+ in. Passive pumps like these are very sensitive con concentration gradients but there are also pumps like the Na+/K+-ATPase antiporter that pumps Na+ and K+ in while spending ATP energy. Since it spends energy it can work well against the concentration gradients.
When carbon dosing is used to reduce NO3 and PO4, it causes bacteria to import large quantities of K as well. And that K is eventually removed from the system by the skimmer with the bacteria.
I try to maintain K in my system around 400 ppm I do measurements for it about once a month and I have seen it drop below 250 ppm. I dose amino acids rather than ethanol or acetic acid, but it cause similar effects. I dont know for how long and how much the OP is dosing carbon, but a long term high dosage could definitely reduce K (I agree that 100ppm is a bit extreme tough).
Also salifert K kit is very sensitive on the dropper used for titration. I would wipe the tip of the dropper after each drop to ensure equal drop sizes.
