CO2 and pH changes

Hello all,
  I have had a 20 gallon tank set up for a few months and have recently 
undertaken measures to treat my plants a little better.  I have increased 
the lighting and begun using substrate and liquid fertilizers.  This 
week, I decided it would be fun to try DIY CO2 injection.  So, I have 
this setup with an old 2-liter bottle discharging through an airstone.  
  My problem is that I have very soft water coming from the tap.  The Aq. 
Pharmaceuticals hardness test shows about 40 ppm and the city water 
report is consistent with this.  So I am getting pretty large pH swings.  
With any flow greater than a few airstone bubbles a second, I get a pH 
drop on the order of 0.6-0.8 points.  I have been adding a little bit of 
baking soda to correct this, and am able to reduce this change to about 
0.2 points.  I don't mind doing this, but want to be sure that it's not 
dangerous to my aquarium in non-pH ways.

  The tank is 20gal, lighting is one 24" GE plant and aquarium and one 
Phillips Daylight fl. bulb.  I have five 3" angelfish, two corys, a clown 
loach, and three otocinclus.  Because of the angels, I don't want to use 
a remedy which increases the hardness, if possible.

  Thanks for any advice.