Well, aside from the diffuser popping off the tubing yesterday evening (and scaring me, let alone the fish!), CO2 levels are OK again this morning, a nice green colour on the permanent test. I currently have the diffuser in a place which seems to have a circular current...hard to describe (it is before a bit of mopani wood but after the filter outlet) which means the bubbles are in the water for ages, moving in a circular motion instead of floating straight up to the top. So far so good, though I can see me either zip-tying the tube ends onto the diffuser and bottle or getting some aquarium sealant and glueing things together.