It'd get very acidic if you added a lot, for one thing. Since there are so many things dissolved in the water used, I would think that the lower water content outide of the fish's bo444444444444444444444444444444444444444444444444444444444444444dy would probably result in the fish losing copious amounts of water and dying. (sorry about all the 4's, but my hammy wanted to type, too, so it'd be rude to delete her work. )
Of course, the high CO2 (well, it says *carbonated*, not sure if it's carbonate or carbon added... methinks carbon since c+ 02 = co2, but that's not by any means the case...) levels and would kill the fish, or the bubbles might form in their fins or gills, burning them.