I used to make diffusors for CO2 like that... find a branch of a suitable tree (I used Linden tree Tilia for that) cut off the outer layer (bark), let it dry a couple of days, cut it to about 1/2 in length, and stick it into an airline. Position this as low as possible (close to the substrate) in your tank. Once the pressure builds up, you get very fine CO2 pearls which mostly dissolve on their way to the surface.
There can be quite some pressure in this system, and the wooden diffusors need to be replaced once in a while. But nothing beats the size of them.
On the other hand, if you do use a reactor, you can just use any diffuser or let the bubbles bubble into the reactor. I imagine it can be easily overdone in a tiny "tank" like this one.
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