Effects of Water Changes

Hello People,

I've been struggling to make sense of what can be causing the 
following ...

I have a heavily planted 20 gal tank lit by a 24" vitalight and 3 
13W compacts. I'm injecting  CO2 via yeast. My plants are doing 
great and algae is there but not yet serious. My tap water has a PH 
of 7.0 and with CO2 injection I'm getting it down to 6.6 to 6.8. I 
can't argue with the results as any more growth and I'd have a 

What I find very strange is the what happens when I do a water 
change (which I do, twice a week). I do a 20% water change (very 
soft water with few trace elements, I've been told by someone who 
got the water tested). Immediately (and I mean within minutes) 
following a water change, photosynthasis (sp?) is like super 
charged. Bubbles pour out from plants throughout the aquarium! This 
continues for about a day and each passing day slows down until 
very few bubbles are being produced. Another partial water change 
and the cycle repeats itself. 

My question is how is it possible for this kind of instant response 
to take place? Although there is only a little amount of chlorine 
added to our water supply, I use Instant Chlorine Remover from 
Aquarium Pharm. with each water change. Can this bring on this 
phenomenon? Is it possible there is some kind of trace element in 
our water that is quickly depleted between water changes and its 
presence would cause this kind of instant response? Do plants 
respond this quickly normally when a missing ingredient is added? 
Is it possible that the sudden jolt in PH change (although not much 
of one) would cause this? I find this so curious and would really 
like some insights if anyone has any ideas.


Cheers, Rene