Bubbles during water change

I wanted to thank everyone who responded to my post concerning the explosive formation of oxygen by plants after my water changes.   I thought it may help to add a bit of information .  In my case there is little or no additional agitation of the water's surface during water changes.  My filtration inlets and outlets are located in the floor of this 150 gallon tank, approx. 2 feet below the surface ( 1 foot after a 50% water change).  I have not noticed any activity prior to the addition of the replacement water, which consists of 1/3 R/O and 2/3 tap water from a well (450 ppm GH).  In fact, the activity slowly increases to a peak approximately 3-4 hours after the addition of replacement water.  I have not noticed any signifigant differences in optical clarity, but will pay more attension next time.  Paul Sears's idea that a calcium deficiency may exist is an interesting one, which I hope to test over the next week or so.  I will begin adding CaCO3 to determine if I can duplicate this effect between water changes.  Other comments and idea's are welcomed and appreciated.

Scott Corbeil