I also am having this problem...same scenario...can you show or explain how to get the skimmer intake outside of the cabinet please ?Your house is new so your HVAC is tied to the outside air, so indoor CO2 should not be your problem. Your tank design however ... well that is your problem. I have a real problem with enclosed sump areas and this is why. Also if the top of your tank is enclosed then your tank has no way to "breath". The CO2 is going to build up. A better alternative is chaeto or macro in the sump. Those mangroves aren't going to cut it they don't grow fast enough. Also uncover your tank (if it is covered) and get more air flow through your stand. Just leaving the doors open should be enough to pull up your PH. Another alternative is to put your skimmer airline outside of your stand. These are much cheaper and more effective solutions than a CO2 scrubber.