If this is your bedroom tank I'd tend to agree with Brandon. CO2 is heavier than air so it will concentrate in the lower areas of your home, not your upstairs bedroom. Unless your house is tight as a drum and the HVAC is mixing the air column in the house.
It's not quite as hot today and it might be a good time to crack a window for a few hours in the bedroom in the bedroom when you get home. See if you get even the slightest bump.
I don't think my little mod is going to help you. My sump sits right on the floor of my basement and the intake for the skimmer is less than 30" above the floor. So I just ran a hose up above the tank level, a little over 6" up. As we get closer to winter when we need to turn the heat on I'm going to run a line outside.
Unless it falls below 7.8 I wouldn't worry about it. If you're that concerned you could cut off the Kalk and bring up your Alk with soda ash. If you look at my PH you'll see where I dosed 2-part Alk last night... I haven't had any coral in my tank until this last weekend and what I have now (Thanks to Ricky) is very small. So I have only been checking Alk every couple weeks and adding what the bacteria uses.
