I have an MRC CR5 reactor, recently I found that if the effluent can't flow,clogged or off, it will backup the chamber with co2 . This has happened twice even after a cleaning.
Yeah this is why I pretty much run mines at a higher flow from the effluent.. Causing my Cal and Alk to be higher also the Mag... Only affect I have seen since is increased coral growth.. About a inch every 1.5 weeks...