tap water CO2

> > This very same bubbling occurs in all 3 of our plant tanks every time we
> > do a water change - new water is 100% DI reconstituted and stored for 1
> > - 3 days before use.
> The same thing happnes in my tanks after a water change.  Does this 
> happen to everyone?  It seems like it does. In the latest issue of FAMA a 
> man said he has this happen to his tanks.  He tested his tap water and 
> claims it is high in CO2 and its the CO2 from the tap water that causes 
> this.  It was FAMA so I would take it with a grain of salt.  It would be 
> nice to know what exactly causes this to happen.    

I don't know that it is exactly what causes it to happen, but tap water 
and RO water can be somewhat CO2 rich.  Some municipal water supplies use 
CO2 injection to control the pH of the product water.  In some cases, 
this is done after lime injection (to boost the calcium and alkalinity) 
which is most often done to harden the water to "passify" it, or make it 
less corrosive.  This is thanks to stringent lead control regulations.  
By adding lime and CO2, the water is much less corrosive and much more 
likely to form a protective scale layer inside pipes.

RO membranes pass more CO2 than most people would expect, so the product 
can have a fairly high CO2 content.

> > Since our water mix is so soft, 3 GH & 3 KH, I thought that it might be
> > calcium that was limiting so I started experimenting with adding small
> > amounts of CaCO3 to 1 of the tanks with no change noted except a 1
> > degree rise in KH - no change in GH or PH - which puzzles me as I
> > thought that when adding CaC03 both KH & GH would rise equally?

Calcium carbonate addition will raise both the total alkalinity and the 
calcium content (and therefore GH.)  Alkalinity test kits tend to be more 
sensitive than calcium test kits (or GH test kits) so I would assume that 
this is a "test kit thing" rather than a suspension of the laws of chemistry.