I believe, that "seltzer-water" is synonymous for "Selters-Water".
Selters on Lahn is a small town in Hessen/Germany with a famous mineral-water
spring. The firm which bottles the mineral-water uses the town´s name as a
brand-name.
Because of the considerable tradition of the spring, "Selters-Water" is in
everyday usage in Germany an equivalent for any mineral-water.