You did a full run without co2, and it went from healthy to ill?f-e:The co2 i was using is NG.I’ve vented air with it on and off.The one I completely vented it was off,and it’s off as we speak with 300-350 ppm from fresh air coming in.
The 4 weeks turnaround is a while. Was it getting beni's or h2o2 perhaps? I guess it's not air borne if it took so long. Or a virus. Making something in the pot a real possibility. Food/acid/mold/bacteria.