Get an aquarium pump. Make a bong with a quart jar inside your terrarium.
I took the containers top and silicone sealed an oxygen run directly pouring out over my cakes(jar now external). Not a ton of condensation but their loving it basking in that haze of h2o2.

Channeled humidifier tek is still the best tek of the millenium.

hAsTa

thats aninteresting question. I have about 96
half pint cakes in a highrise sealed terrarium. an ultrasonic humidifier pumps fresh air in through the top. then the air naturally settles downward in the terrarium due to the co2 being produced by the cakes. it flows out the bottom over some green foliage. I have measured the intake and exhaust air with a cheap co2 test kit and there is a net increase, albeit a small one. The 96 cakes certainly dont make as much co2 as a human being, for sure. I would say that the actual amount of co2 from one cake is rather neglegable. But in a small soda bottletek, you need to give that cake fresh air as much as you can, so either upgrade to a bigger terrarium with air flow or live with what youve got.

peace