they are replacing some of the salts with antifreeze, glycerol and silicates to get to a point where a freshwater fish won't die. Its a neat idea but who knows what the long term effect is on the fish.
That stuff is meant for use in a brackish environment according to the manufacturer and only works for some salt-tolerant freshwater fishes. It's best used for quarantining saltwater fishes in order to kill off parasites while keeping the fishes happy.
If you want to make a brackish aquarium, then you still have to use salt. This additive doesn't replace salt rather it somewhat supplements it somehow; there's really no use in using it for a full-time brackish aquarium.