The 150g tank was the first time I did CO2 injection. I selected the Aqua Medic 1000 CO2 reactor shown below:
It has worked well, but it's expensive. Anywhere from $80-120 depending where you shop. It's a simple design, a tube filled with bio balls with hose barbs on either end, a straw tube to pump in the gas and a vent at the top to purge air.
I was looking through catalogs and came across this which looked surprisingly similar. And only $35.
I purchased one along with a box of bioballs...
A couple dollars at my local Petco got me a hard tube that air hose tubing fits over.
I opened it up, poured in the bio balls and drilled a hole in the top for the tube:
I carefully threaded the tube through the dividing disc and down the side, stopping an inch or so from the bottom.
Finally, a few drops of super glue finishes it off and (hopefully) welds and seals the tube in. I haven't water tested it yet, but if it leaks, I will add silicone. In this model, both the water tubes are on top. The one entering the middle of the reactor chamber connects to a tube that extends down to the bottom. In traditional use, this would be the inlet, water would be pumped to the bottom, flow up through the media and out the top. In my case, I want the water to flow against the CO2's natural tendency to float upward. So I will pump the water in the top and force it to travel down through the bio balls before flowing up the pipe to get out.
The only thing missing from this is the air vent. I have some small valves from an indoor watering system that I thought about using. They have barbs on them so installing them directly would be challenging. I could mount a small piece of the straw tube the same as the one there already and use a small piece of hose to bridge between the two. Or I can play it by ear and see if I need to be able to vent air regularly.