Seltzer, aka soda water or sparkling water is carbonated water with no additives, carbonated by injecting carbon dioxide (CO2). It's the base for brands like La Croix, or what comes out of a SodaStream. Sparkling mineral water is made with water from a mineral spring, like Perrier or Topo Chico. Club soda is carbonated water infused with added minerals, which give it a salty or lightly sweet flavor. Additives you may see in club soda are potassium sulfate, sodium chloride, disodium phosphate, or sodium bicarbonate.