Thanks Richard, makes perfect sense. Yes, the fill water has high TA. Our "city" water is well water and has high TA and high ph. Since you explained the outgassing problem I completely understand it. (not really a problem... just a new twist).

I really gave up a long time ago on trying to lower the alkalinity in the pool. To lower the ph and keep it low enough long enough for aeration to work, is way too much trouble given that it all immediately begins to rise again from the fill water. Rain water doesn't appear to be much better as an additive.

Thanks for the help.