Ok, so we're going to make CO2 reactors at the meeting. It's based on the design Tom Barr recommends. If you'd like to make your own, this is the powerhead we will be using. It's available at PetsMart and surely other places. You can bring/buy a different one if you like, but ease of integration will be unknown. I'll provide the rest of the materials, so it would be helpful if I could get a count of how many people will be building one. I've used this design in my 72g tank with great success, I'm sure it would work in larger tanks, but I'm not sure how large it could reasonably support.