For potassium, you don't dose for the whole tank each week. That would lead to a steady increase of chemicals in your tank. You can not assume that the plants take up all the nutrients supplied, and since you can't measure it, to be on the safe side you only add enough to bring the replacement water to the desired level.
For NO3 (KNO3-dosing) you should monitor and dose according to your tank level. Fish poop and decaying plants all add to the NO3 level.
For example, if your test kit says 0, you should dose for the entire tank, to bring it to 5 ppm. But you need to be careful and do this gradually, since sudden changes in water chemistry can be harmful for your fishies. Plus the testkits I tried are not very precise in lower levels, between 0 and 10 ppm.