Roots need o2..not co2. You can use some hydrogen peroxide (h2o2) to add some oxygen to your roots. The h2o2 turns into h2o and releases an oxygen molecule. I'm using the 30% stuff and toss a cap-full into my 2 gal water bucket. I just use tap water.

There's not much you can do with the soil your plants are already in without getting your plants all pissed off at you. When you transplant, use a more airy mix. I've used that same miracle grow potting mix and my buckets were like buckets of cement....really heavy even when the soil was dry.

I'm getting really fond of adding coco peat to my soil to make it fluffy and airy. I'm playing with different mixes, but I use at least 1:1 coco to soil. 2:1 coco to soil might be even better.