when i did it, i added the carbonated water (sparkling soda) in 4 oz increments. i cant remember how much i used, i think it was less than a liter though.
i imagine the exact dose would vary. either way, the scuds first darted for cover, then started looking like they were trying to crawl out of the tank, and then started dying. it didnt take very long. just be sure to bubble the water and check the ph to make sure its back to normal before you add your shrimp back in.