downdraft Ca reactor help needed

[vanreefer]

Hey,

Does anyone have any advice on downdraft Ca reactors... My problem is I get a collection of CO2 at the top of the reactor which stops the effluent from exiting the reactor at a predictable rate the pH inside the reactor is typically 6.8-6.9 with about 1.5 drops of effluent/ sec and one bubble/ 2sec of gas
Ca and Alk levels are decent when I am around every day and can make sure the effluent rate stays constant typically Ca 400-420 and dKh of around 10.... But I sometimes (often) I am away for 5 days at at time...
Any suggestions...?

TIA
Dan