This discussion needs to be differentiated.
The important point is not the absolute value of the ec or the Microsiemens-number, because this says nothing about the minerals that cause it. The decisive point is calcium, Ca.
Parosphromenus and other typical blackwater organisms are not very tolerant of calcium. They are not accustomed to it; there is nearly no Ca in the tropical waters they live in. This makes the decisive difference to the bulk of our tap waters. Mostly, Ca is the dominant factor in it. In some cases, however, we have a different, a-typical combination of minerals in tap-waters. In some Black Forest regions, for instance, you have a tap water containing very small amounts of calcium.
The conductibility says nothing about the combination of minerals that they contain. Therefore, two waters with the same ec can be composed of quite different minerals. Parosphromenus stand rather high values of Na, because they are biologically used to it, but not of Ca, because they are not accustomed to it at all. (Think of the treatment of some diseases by salt, Na; they stand it very well).They may live in Ca-waters for quite a time, but they age much quicker than in Ca-free waters.
This explains why there are seemingly some contradictions on a water with the same ec. It’s the same ec, but in reality the waters are quite different. The important point is: For Parosphromenus, try to avoid calcium Ca. Slow amounts maybe tolerable, but in most tap waters the amounts are far too high.