I found a simple to make, cheap method to costruct a power CO2 reactor for 2$ plus the cost of a small power head. You can buy these small part's holder's at OSH/ Orchard's supply called viewtainers which can have the hold needed for the power head melted into th side or you may cut a small hold for a snug fit pointing the power head downward from the top. The venturi is connected to about 2 inches below the top of power reactor to small hold for venturi action of the CO2 when it builds up to this 2"(or any height you desire) size before it begins to waste the CO2 as a mist.
This is essentially what I sold as an internal but this is out of soft acetate plastic and is easy to work with.
You bubble the CO2 in from the bottom and this acts as a bubble counter.
A lighter can be used to burn a hold into the plastic if you want a side position for the power head or to make holds for suction cups for top mounting of the powerhead.
I'll post some pics later, but at 2$, it works as well and is slightly smaller than anything else and if anyone can beat the price, well, you might want to sell them.
Here is the part: http://www.viewtainer.com/
You can use my old design or copy other internals off the web. Most DIY'er get the idea with these pics.
Adding the venturi loop will help increase the efficiency when you need it and waste CO2 when you don't when the tank has already built up to saturation levels of CO2.
These work very well and are easy for the beginner to use and are also very good for DIY CO2 users, add a small hole(3/16") opposite the power head side or about 2.25" down if you use the venturi loop at 2" for burping when the power head is shut off at night(for semi automatic control).
Holes to make:
Hole for the power head inlet(Make it smaller than the powerhead's pipe outlet, then use a lighter to make it slightly smaller so you can get a very snug tight fit/silicone it/epoxy etc)
I like the side mounting personally but top mounting can be done and you will need a sponge to prevent the water from flying out oif the bottom or you can use an elbow(better))
3/16" for venturi loop 2" down from top
3/16" burp hole 2.25" opposite venturi
3/16" CO2 "in" on top of tube for CO2 bubble input(you may wish to se a 4" piece of rigid 3/16" airline tubing for this
Hey, it's 2$ and you can try it.