Measuring conductivity is not something to worry about or fuss over, fortunately [provided we are doing water changes as noted below]. We have enough of that already in an aquarium.
As that linked Fluval article mentioned, conductivity just measures the TDS (total dissolved solids) in aquarium or habitat water. You will frequently come across "conductivity" rather than "hardness" in articles about natural habitats. Most of us think of hardness as GH and KH, caused by various minerals. And these are extremely important for fish. But so is the actual water in terms of the TDS which are related but not necessarily minerals.
As I mention in the article I recently posted on hardness, and again on salt, TDS get into the tank from all sorts of places. Fish food, water conditioner, medications, treatments, plant fertilizers, any chemical substance... etc. These too affect all fish. But fortunately we can control them easily by regular partial water changes. [obviously, this topic will form a major aspect of my forthcoming article on water changes
.] We can't help adding TDS via conditioner and fish foods, but with regular water changes we remove them so they do not continue to build up.
You mention knifefish, and obviously such fish would be sensitive to TDS. But the water changes still deal with these adequately.