Does plain seltzer need a hechsher?

Answer: No (cRc).

Reason: Plain, unflavored seltzer and club soda are recommended even without certification.

Source:

• cRc: plain, unflavored seltzer is “Recommended”; unflavored club soda is also “Recommended.”

Related questions:

• Does flavored seltzer need a hechsher? Yes (cRc).
• Does bottled water need a hechsher? No, if unflavored (Star-K, cRc).

References: cRc (askcrc.org), “Seltzer Plain, Unflavored” · cRc (askcrc.org), “Club Soda Any Unflavored”