Liquid fabric softener and black/gray tank smell

Won't hurt anything, but I don't know effective it will be. Might mask the odor for a while. Whole idea of the water softener is to create a slick surface inside so that things don't stick to it. Another part of geo is to use some bleach when odors occur. As mentioned, your tank is a storage unit, not septic, so bacteria, of which odor is a by-product, is not necessarily your friend. Bleach kills the bacteria that creates the odor.