Brian,
City water is treated with chloramine which is a compound made up of ammonia and chlorine. Prime will neutralize the chlorine and convert the toxic ammonia to a non-toxic ammonium. Prime will not do anything about nitrate, though.
I don't know enough about water softening systems to offer firm advice, but I don't think I would use the water you are describing in my tank. An RO or RO/DI system with this type of water will probably have you going through membranes like crazy. Any chance you will buy a house in Edmonton at some point where you know what's in the water?
Here is Epcor's SUMMARY OF MAJOR CHEMICAL, MICROBIOLOGICAL AND PHYSICAL PARAMETERS OF EDMONTON DRINKING WATER for July 2004 (in an Adobe PDF file):
https://www.epcor.ca/NR/rdonlyres/72...072004_211.pdf