A question about seltzer and club soda.

You can, in a pinch, substitute one for the other, but club soda as jwolfsthal said, does have some sodium added

Here's a bit about making DIY mineral waters with the sodastream. I've tried some of the suggestions just a light touch of baking soda, and salt, and a few flakes of Epsom salt.
And it did give a nice fuller flavor like you'd find in bottled mineral waters.

http://blog.khymos.org/2011/01/30/diy-mineral-water/#more-2331

spot on answers...for a refreshing hot weather drink try a "sonic"...half soda half tonic with a lime! mmmm!

I can taste the sodium in club soda and prefer seltzer. I lived in Toronto for awhile and at the time it was hard to find seltzer, except in speciality stores, and I bought all of the equipment to make my own!!

Club soda has sodium bicarbonate which (in my personal opinion) gives the bubbles a harder stinging fizz, Seltzer merely has carbon dioxide dissolved into water under pressure, which bubbles out when you open the bottle.
You can make out a slight difference in taste when you leave a bottle of each open to go flat.

Exactly right.

club soda usually has sodium added, while seltzer is usually just CO2 carbinated.