Here's what Tom Barr has said on this issue of CO2 in tap water.
It's entertaining to suggest a myth vs fact on such topics.
As someone who has a water treatment operators license, I can tell you that most tap has more than ambient, and a lot of tap water is really loaded, well/spring water in particular.
I'll present my case easily here:
Caves, you know, the places folks crawl around in, my Dad was a cave biologist.
How are they formed? CO2..........
Why would ground water be loaded with CO2?
As rain falls, it enters the soil, bacteria give off XCO2, that's absorbed by the water, but there is nowhere for it to off gas. So it builds up and becomes rich in CO2, this eats away at limestone, which is a nice base that reacts well with CO2=> caves are formed.
Typical CO2 ppm in spring water: 10-30ppm in Florida, KY, IN, OH, AL, NC, SC, IL, MS, AR, TX, NM, CA, MN, WI, MI, NY, PN, WV, VA, TN..... anywhere there's limestone and many places there isn't. Not all tap will be loaded, but __most__ is. Ground water is almost always much higher.
Why would cold tap have more CO2 than your warm tank water?
Water at a colder temp holds more gas than at ambient conditions.
All gases...............
Those are the "facts" about tap water.
It said in PFK somewhere, by Jeff Walmsey.