is there a case on a non-injected tank to have to much agitation to reduce
CO2 than would otherwise? (meaning, fish giving off more CO2 in the tank and
if you agitate it too much it would actually take out the CO2 from the
water)?

No.

or is fish respiration never great enough to create that kind of
imbalance/supersaturation in a soft, alkaline water?

Yes. Fish respiration produces negligible CO2 compared to diffusion with the
atmosphere.

Phyl told me not to use bubble agitation (venturi), as that will cause the
CO2 to escape too much (she knows I don't have CO2 injection)... so I'm
just wondering...

Either she's mistaken, or she forgot you don't have CO2. There's no reason to
avoid aeration or surface agitation in a non-injected tank.


Leigh

http://www.fortunecity.com/lavender/halloween/881/