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A Simple CO2 Reactor

(Revision A)

I have been using the "pump intake" injection method for CO2 for two years with adequate sucess. This involves running an airline from the bubbled CO2 to the intake of a cannister filter or powerhead. The impeller breaks the gas into little bubbles, a certain fraction of which dissolve into the water.

This was fine on my previous tanks. However, my current planted tank is larger than my earlier ones, and uses a prefilter and sump, so a certain degree of churning is apt to take place, liberating precious amounts of CO2 back into the atmosphere. In having to refill the tank for the third time in 9 months, I decided enough was enough and built myself the cheap gas reactor described here.

The reactor's operation is simple: Water passes through the reactor, trickling over the media (bio-balls, shotgun wadding, or in my case, ordinary aquarium gravel), while CO2 simultaneously bubbles up from the bottom. This gives the CO2 in the chamber ample time to dissolve into the water before it leaves. It is assumed that all the CO2 dissolves, so the CO2 level in the tank will be dependent on the bubbling rate and how fast it leaves the tank water.

Here's a sketch of the unit I built.

The reactor is constructed from a 10 inch section of 1" PVC pipe. At both ends is a PVC el with one side threaded, and screwed into the thread is a drip irrigation manifold. I glued the els to the length of pipe, but only screwed the manifolds into the els. This way, I can change or clean the media later. I used the drip irrigation manifolds because they were cheap and an easy way to get 1" PVC down to 1/8" airline tubing. A hole has been drilled in the pipe as close to the bottom el as possible for the water to exit back into the tank. Finally, I filled the reactor with large gravel because I didn't have anything else handy at the time. I am getting good enough results that I will not bother to change this.

The reactor sits half-submerged in the sump. I tapped off part of the tank return from my sump using a drip irrigation tap, and ran that to top manifold using standard airline tubing. Through the bottom manifold comes the CO2, also through normal airline tubing. (Some say this will get brittle and break eventually. Hasn't happened to me in two years yet, but I have some silicone tubing ready if it does.) The water returns to the sump through the hole in the side of the pipe.

Initially, I built the reactor with both CO2 and water entering the top. However, it was pointed out to me (thanks Booga and Dan!) that injecting the CO2 from the bottom is a better approach because it has more of a chance of dissolving. When I switched to this method, and had the water exit through the bottom manifold as well, I found that the exit water pushed the CO2 right back out, undissolved. Hence the need for the small hole drilled in the side of the PVC pipe. :) It does what it should now.

I erred on the side of caution when calibrating. Before hooking everything up, I stuck the CO2 tube directly in the sump to observe the bubble rate was 1 per 3 seconds or so. Then I hooked it all up and adjusted the level slowly over one-day increments to get the level right.

So does it work? I would estimate I went from a bubbling rate of 5/second with the powerhead injection method, to somewhere around 1 per second using the reactor. I'd call it a success!

Cost: two manifolds ($6), misc PVC ($2), teflon tape (had it already), drip tap (had it), rocks (had em) = under $10. If I were really ritzy, I would have built the main tube out of clear PVC or acrylic, but who cares!

Erik Olson
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This page was last updated 29 October 1998