For all of my fish aquariums--bar one--I use salt. The aquarium in which I have my cories is one in which salt is used, likewise with the aquarium in which resides a plecostomus. Not only have these fish, for me, survived, but they have thrived, especially in the case of my plecostomus who appears to be growing at quite a startling rate--as fast as, if not faster than, the plecostomus in whose aquarium there is no salt whatsoever. Thus, I believe that the question is not whether to use salt or not, as many people would say, but as it has been put in this thread--exactly how much salt is safe?
For my twenty gallon aquarium, I use one (meager) tablespoon of salt per ten gallons of water. For every one gallon of water, I use 1/4 teaspoon of salt, and I am always careful not to overdose... This amount of salt appears to work well for most of my fish, exempting the molly, for which a greater percentage of water salinity appears to be needed for its health. Thus, I would recommend 1/4 teaspoon per gallon for your aquarium, perhaps slightly less.