I recently added a pressurized CO2 system to my 38 gal aquarium. I use
RO water and added calcium carbonate to obtain 4 KH and 4 dGH. The pH
initial shot up to 8.5 (from 7.2) and then dropped to 7.6 overnight.
When I got home from work it was down to 7.4 and has not moved in the
16 hours since then. Is this typical?

Here is the system:
Aquamedic CO2 Reactor 500
Milwaukee SMS122 pH Controller
Milwaukee MA957 Regulator
Eheim 2011 to provide the current

The 2011 and an additional 2213 are using 2 Eheim surface extractors.
Neither filter's discharge is directed at the surface. There are no air
stones either. So there should not be much CO2 stripping going on. I'm
using a 96wt 5700K compact light and the tank it moderate to heavily
planted. Substrate is a quartz sand/laterite mix.

Is there any obvious reason the pH is being so stuborn?