Is your softener one that uses salt? Perhaps some of the chloride ions are reacting with the Purigen.
I would not use water that has been through a softener. I am surprised your tds is so low. I always was led to believe there would still be some sodium in the water after the softener treats it.
No idea how to test that. I know when I lived in a house with a softener, I would use untreated water, just to be safe. No science, just precaution.
So, is softened water definitely bad? I don't remember seeing anything on Seachem's site or FAQ mentioning softened water, or salt for that matter.
As an update...today I rinsed the bag again, and placed it atop the filter pad in the filter. But, this time I stood the bag on end, with only the short side of the bag in contact with the pad. There was only about 1/4" of product in the water flow. Within 5 minutes I had to remove it as all the adult fish were again at the top of the tank mostly motionless, and pretty much stuck in the plants.
Either I have bogus product, or something in Purigen does not like my water!!!
I'm going to post on Seachem's site. Maybe I can get swap from them to make sure that what I have is legit. Or maybe they can suggest something to test for...