Well, I've got it all the CO2 equipment set up but I'm having trouble keeping CO2 levels up. The Mazzei injector work fine. I get a nice mist of CO2 throughout the tank.
Even with a continuous stream of bubbles I struggle to push pH below 7.0 (kH is around 7) and my test indicator only just turns green. If I turn off CO2 the levels fall rapidly whereas my corner tank takes hours for the level to drop.

My suspicion is that the sump is gassing off in the return chamber. The weir has a minimal fall and I get little disturbance but it has about a four foot fall and I find the water foaming quite a bit. I just can't stop the return pipe sucking air down with the water. In the weir is a Durso style arrangement.
The pipe is a 1¼" through the bottom of the weir and into the sump via two 45 elbows. At the sump water level I put an elbow in and run it along just under the surface for 10". I've tried several arrangements but can't minimise the disturbance. If I run the pipe to the bottom, the air bubbles up so wildly it drives water over the top of the sump. You guys have far more experience than I do and I like to hear your suggestions