Back when I had to winterize, about 13 years ago, I would get the fizzy water after I dewinterized. It would last for awhile but it soon went away. I would have the water heater drained, and no antifreeze in it, and would fill it with fresh water and flush all the lines real well. I assumed it was caused by the anti-freeze, but didn't know for sure.
Roger & Cheryl Full Time since 2002, RVing since 1975 2008 Chevy 3500 Crew Cab Dually, 2012 RW 36RL