Carbonated water is water plus carbon dioxide, kept under enough pressure that the CO2 stays in solution in the water. When you open the bottle, CO2 starts escaping from the water. When you pour it into the tank, more CO2 escapes. What's left is CO2 added to your tank, for the short time before it all leaves from the water surface. You could kill the fish by adding too much at one time, just as you can kill them by adding CO2 any other way. Amano is said to have started using CO2 by using carbonated water.