Yeah, the durso is just expelling the air bubbles+whatever CO2 diffused out as the air/water fell through the pipes. A herbie oughtn't do that. I don't really care that much that the sump is wasting gas, gas isn't all that expensive compared to all the rest of the monthly expenses. Just increase the gas to the level that works for your plants and animals.
I agree. I don't know much about the Herbie other than what I have read the last couple of days, but by using this method instead of the vented Durso and maybe also positioning your overflow exit pipes in your sump under the water level in a manner to prevent as much agitation as possible wouldn't that come close to preventing significant Co2 loss? Someone please correct me if I'm not seeing something.