Dual venturi DIY External CO2 reactor


New Member
Jul 21, 2018
Seattle, WA
I think I could benefit from a reactor like this... Right now I am injecting CO2 into my canister intake and I don't think it is very efficient.

The thread is so old that many images have expired, and I don't yet understand how the reactor operates, all I know is I really like the idea of a HOB unit that does not require me to fiddle with my canister. If anyone can post pics or videos, I would appreciate it.

Andrew T

Jul 15, 2015
Resurrecting an old thread to ask one question.
I have drilled the Venturi to the side, 2” from the top.
Tubing and irrigation drip valve is connected .
Should I let the valve slightly open to allow a drop of water a second to come out of the Venturi or should I just have it closed and release it at the end of the day for maximum performance from the reactor?


New Member
Jul 12, 2019
I though I would share the reactor I built based on Tom Barr's design with the venturi port. All is working wonderfully.
Initial built:


Additional purge ports with different depth added. if I had to redo it I would use the same tubbing size everywhere and use rigid tubbing. I just couldn't source it where I am. Tubbing below is smaller than the standard air tubing (~6mm).



This is the DIY venturi port. Parts glued with epoxy. Try using an epoxy that is not too hard else it might crack when connecting your tubbing. Also don't forget to sand down anywhere the epoxy will contact the plastic to improve adhesion. I also shortened the small connecter and did a ~60 degrees angle for more efficiency. The hole should face away the current.




Venturi port in action. Don't pay attention to all the bubbles in the reactor. That is only a lot of air due to the reactor not being fully purged at the time of the testing.

Reactor in place. The unused purged ports were sealed by melting the plastic as I was having some leak. I will probably never need them for this tank but they are there just in case.

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Resurrecting this thread a bit.
I have 6 display tanks (20 gallon longs) on a central system. I measure pH in the common sump... and inject CO2 into a reactor (ISTA I-539 INLINE) after the sump pump (Active Aqua 1000). Of course this reactor is small and restrictive, so I have several parallel paths after my sump pump so that the individual devices (CO2 reactor, flow meter, UV Sterilizer) don't slow the overall flow. This is complex... and with all of the bends, unions and so forth, I don't quite have the flow I want. My goal is this (pardon the quick sketch.... my Fushion software won't fire up):

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Leave the Pump with a 1" line....Go into top of Reactor with a 1" line (a gentle sweep)... Exit bottom of reactor with a 1" line and make a 90 turn to the UV Sterilizer and enter the bottom of the UV sterilizer with a 1" line (UV Sterilizer is 3" internally), then exit the UV sterilizer out of the top with a 1" line and turn 90 degree to go through a 1" axial (less interference) flow meter.... then split into two parallel trunks to feed the aquariums thorugh 3/4" lines.

To turn over 6 20 gallon long tanks at a decent rate that are effectively in parallel with one another, the Pump will need to maintain a good 600gph flow from the sump and through the equipment. So here is my question. If I take Tom's design and increase the diameter to 3" and the length to 24" and add bio balls.... (and the additional bubble catcher line ... venturi... back to the intake side of the Active Aqua 1000), will I get a decent dwell time so that I can dissolve the CO2 sufficiently?
Finished my CO2 Reactor. It's a bit different than I cited above. Final reactor chamber portion measures 2" x 25"... but the entire thing stands about 3' tall. Unlike Tom's it has a bypass so that I can slow flow through the reactor chamber for better mixing if needed, without slowing down overall flow. I am not powering by a 2nd pump.... but rather my unit is fed from a 1" line from my powerful sump pump. I'll post more photos and comments once it's operational.

I like the design. I will comment on 2 two things:

1 Needs Work:

The rigid airline poked thorugh and glued is a bitt too flimsy. While attaching the airline, I wiggled it too much... and after a few hours of runtime, developed a very, very slow leak. So, I'm installing 3/16" brass barbed splicers. Rigid airline will attach to the splicer with a 1/2" section of soft airline tubing... on both sides of the reactor (inside/outside).

Here's the fitting upgrade..

So, I'll make two observations regarding this DIY CO2 Reactor.

2 Perfect:

It's so quiet. No whistling, no splashy noises... just silence... With my high flows, it drives off the collected air on startup in about 15 minutes.... just clear, silent water.. (I like that I employed a bit of clear PVC so that I can see what's going on inside of it!!).

It's a very good design. With my increased flows, compliments of 1" plumbing, I saw my pH drop .5 points in about 2 hours. Which I thought was incredible fast accross 5 tanks and my dual chamber sumps (effectively 7 x 20 gallon long aquariums). Picked up a new 20 gallon long today, so preparing to add the 6th and final display aquarium to the system.... My pump, filtration, reactor and UV are all scaled for it!!!
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