Problems wigh DIY CO2...

	I have two planted 55 gal (All Glass 48"x12"x12") tanks.  They each
have a small school of ottos and 6-8 juvenile angles in them.  My tap
water is very soft, has a Kh of less than 2, and Ph of about 6.8-7.0.  The
tanks are both lit with 80W of flourescent light (1 triton, 1 vita-light).
I use duplaplant and the dupla 24 drops.  The growth is ok and you can
normally see O2 bubbles coming from the hygrophilas and echinodorus most
of the day.  I have been using DIY CO2, but it only lasts 5-7 days before
it needs to be changed.  Also, due to my soft water and its low Kh, I've
had to use baking soda to increase the Kh to minimize Ph fluctuation. 
Okay, here are my questions:
	o my light is only moderate, is the CO2 at these levels even
helping things enough to justify its use and the instability of Ph it's
causing at this time?
	o I can't afford a "real" CO2 setup, if I continue using DIY what
can I do to eliminate Ph fluctuations and stress to my fish and plants?
any hints or ideas would be greatly appreciated.