your tank is probably testing fine because you just put water in the tank and there is nothing to foul it up. adding stability to your tank won't do anything unless there is an ammonia source, so if the tank doesn't have any fish yet, you are wasting it. if you have a couple days, what i would do is go to the lfs and grab a bunch of cheap fish, ask the guys who work there if they have some established gravel or bio media that you could get from them, if they give you some, put it in your filters when you get home, and drop the fish in the tank.
after the fish are in, you can follow the instructions on the bottle of stablity for a new tank and start from day one. i wouldn't perform any water changes on the tank while it's getting cycled unless your tests say that you need to. when your natts arrive, there should be a good amount of bacteria already built up and you shouldn't have too many problems.
i wouldn't just dump the fish in when they arrive without doing something to help speed up the cycle, piranhas may be tough fish, but i have talked to many people in just the last few months who have had their piranas die because they put them into a tank that wasn't properly cycled or they did something to create a mini-cycle.