I have a GEO 618 and did have the problem with gas building up in the chamber but that seems to be more related to having too fast a bubble rate and not enough effluent drip. I don't have the problem now that I have everything set properly.
You asked for advice so here is mine . . . don't get nervous or sweat setting up the reactor.
Get your results where you want them with additives.
Set a slow bubble rate and slow effluent drip and run for a day or two.
Test again, get your parameters in line.
Adjust the settings faster or slower
Repeat.
It's not like you are going to nuke your tank if your settings are off.
-Mike