H2O.NaCl + CO2 ==> something acidic? Yes, though until you get to extreme concentrations you can ignore the NaCl part.
CO2 dissolved in water is carbonic acid.
Agreed, see post 19. However, carbonic acid is not stable in an open system, and especially with the introduction of heat to the system. Also, rainfall will decompose any carbonic acid on the surface of a body of water, right back into CO2 and H2O.
My original post was poorly phrased; the question is why acidity increases in an OPEN system, not a fish tank or a glass of water.