Something is obviously off with those readings. If the values are accurate, then either the water in/out is off, or the probe is off, as you can't have 6.0 going in, coming out at 5.7 and medium is 7? That doesn't add up. Again, measuring runoff can be unreliable, those probes are for sure not reliable either for testing ph of medium.
Either way, again, if this was related to ph, then you would have a lockout of nutrients, which would be evident on the leaves by deficiencies. Your leaves are fine in terms of color, the cupping appears to be too much nitrogen for now, but was that the case all along? i can't tell since it can't be fully known what concentration (ppm's) you have been giving up to now in feedings.
I saw the small space you're growing in, it's why i said drainage..not necessarily that water doesn't drain out the bottom, but rather how fast the medium dries out. You're watering has been off by giving small amounts and too often, and then in a small space, with low lighting, the plant doesn't drink that much and air movement is minimal as well.
As for your worry about co2, with low lighting, the co2 that is naturally around, is enough. The plants don't need more, as they don't have the necessary light to photosynthesize and take advantage of the extra co2. Plus, you only close the doors in the dark...plants use oxygen and release co2 in the dark..only during lights on, photosynthesis, do they take in co2 and thus release oxygen.