There is no benefit to any type of salt in a freshwater aquarium on a general basis. You can read why in this article, and also learn what this is doing detrimentally to the fish:
Catfish are sensitive to salt, more than some other fish. A while back we had a member who posted photos of a pleco who was actually burnt by salt in there water, and was slowly dying. Removing the salt via massive water changes got the poor fish back to health, though the patches on its body were a sorry sight.
While Stability is not (or should not be) problematic, I would wonder why it is being added. In an established tank this is not necessary, as the colony of nitrifying bacteria will be capable of handling the nitrogen. Or it should be, if the tank is not somehow compromised biologically.
The nitrates is also an issue, and may have contributed. It is now known that nitrates above 20ppm do affect all fish to some extent, and here again catfish are in the group of sensitive fish that will be more adversely affected than some others. I would look into the nitrates; are they in the tap water at all? If not, then inside the aquarium one has to consider the volume, fish load, water changes, live plants.