Using a 5-year old Up Aqua in line atomizer hooked to outflow of an Eheim 2075 at 100% flow:
Tap pH 7.2
48-hour de-gassed pH 7.1
At co2 off pH 5.5
7.1 - 5.5 = 1.6 pH drop.
Rather surprising but the tank also has a fair number of transitioning plants with some melt and new driftwood. Adjusted co2 down and test again tomorrow. Personally, I want to bottom out at around 6.2. I feel that I will be more lifestock limited then by a specific diffusion method. If and when I switch over to sump, the out-gassing is expected to be higher.
In my 60P with in tank diffuser, the pH drop is 7.1 - 6.7 = 0.4 pH drop, lifestock limited @ the current surface agitation and no surface film.