Inline CO2 Diffuser Review

Our aquarium’s first pressurized CO2 system used an in-tank atomizer to disperse the CO2 in the tank. In-tank diffusers work very well, but they involve having some added equipment in the display section of the aquarium – the diffuser and some CO2 tubing. I have always considered trying to get an in-line CO2 diffuser installed (and hidden) in the pump section of our Spec V. The challenge in that application is finding a model that was compact enough to fit in that small space.  I finally found one that fits and decided to give it a try.

Inline CO2 Diffuser - packaging from CO2Art

CO2 Inline Diffuser – Description and Dimensions:

The inline diffuser I purchased was through a web vender,, out of the UK.  They were the only vendor of this diffuser at the time.  The cost was $40 USD; when you add cost of shipping from the UK to USA, it was near $50 USD.  I have sense come to realize this exact device is sold by numerous vendors and they are exactly the same.  Here is a source selling the 12/16 In-line Diffuser for $26 USD.

In-line CO2 Diffuser - 12/16 size

The diffuser is available in two models that vary in size, primarily for the main inlet/outlet tubing size.  I got the 12/16 version.  This means that it takes tubing of size 12 mm Inside Diameter and 16 mm Outside Diameter.  It just so happens this is the size of tubing from the Spec V’s outlet tubing – Sweet!  It is also available in a larger 16/22 version.

components of Inline CO2 Diffuser

The device has three connections – the two aforementioned water connections (water in and out) and (1) smaller nipple connection for the CO2.  The concept is to install this in the return tube between your pump and the ‘return’ outlet in your tank.  The CO2 enters the body of the device and the gas gets forced through the ceramic to create fine bubbles that are injected into the passing water stream.

The device instructions state a minimum 30 psi of incoming CO2 pressure is required.

Measured dimensions of this in-line CO2 diffuser (12/16 version) are as follows:

  • Total Length: xx” (xx mm) long.
  • Body Diameter: xx” (xx mm).
  • Width at Widest (at the CO2 connection): xx” (xx mm) long.
  • Water Connection: 12 mm Inside Diameter and 16 mm Outside Diameter Tubing
  • CO2 Connection: standard CO2 tubing (1/8″ ID and 1/4″ OD)

In-Line CO2 Diffuser – Installation and Use on a Fluval Spec V:

This review is somewhat limited in scope as I only tried it out on our Spec V aquarium.  I will first say that it was a very tight fit to get this installed in the pump section of the Spec V.

I used the stock outlet tube and carefully cut a section out to mount the CO2 diffuser in-line.

cutting the return tubing on Spec V aquarium to install Inline CO2 Diffuser

To make the water connections, you fully remove the threaded collars and install on the tubing.  Next, install the tubing over the barb fittings on the device.  Last, slide the collar down and tighten.  Do this for both sides.

Inline CO2 Diffuser installed with tubing

I performed the flow tube modification of punching a hole in the return tube (below the CO2 diffuser).  This lets some water mix in the pump section so that the heater operates correctly.

flow tube modification for Spec V Aquarium with Inline CO2 Diffuser installed

Installing the CO2 tubing is a similar process.  CO2 tubing is notoriously stiff, so consider heating the end in hot water for a few minutes before pushing the tubing on the barb.

CO2 tubing connection to Inline CO2 Diffuser

Finish by screwing on the collar to secure.

Inline CO2 Diffuser installed in Fluval Spec V Aquarium

This is where things got challenging to install in the Spec V’s pump section.  The CO2 connection comes out the side and make fitment very tight.  I had to sharply bend the CO2 tubing upward coming out of the device.  If I positioned the CO2 tube on the diagonal (in the corner of the pump section) it just fit.

Inline CO2 Diffuser installed in Fluval Spec V Aquarium

Last items for installation is to reinstall the outlet nozzle and make all the connections to the CO2 system.

Inline CO2 Diffuser installed in pump section of Fluval Spec V Aquarium

First impression were that the tank looked much better with the old in-tank diffuser and CO2 tubing out of the display section.  In that regard the product is a success; it provides a very clean solution.


Performance, with regards to CO2 dispersion, is where the weakness of this product lie. Right off the bat, the bubbles produced and dispersed out of the return nozzle were very large.  I got feedback from reading forum experiences that the ceramic needs to break-in a bit before the bubble size reduces. I did notice the bubbles got smaller after a few days; however, they were still much larger than what we experienced with the in-tank bazooka diffuser.

With the old bazooka ceramic, the bubbles were very fine and were dispersed throughout the tank.  I could see tiny bubbles floating all around – even near the bottom.  With the new in-line CO2 diffuser, the bubbles were large and did not go all around the tank. The stayed near the top, floated up to the surface, and went bye-bye.  I was hopeful that despite the different look of the bubbles that CO2 was still being dissolved in the water column.

It turns out this was not the case.  The health of our planted tank started to suffer – slow growth and more importantly an algae outbreak.  Telltale signs of CO2 problems.  I tried to compensate by increasing the CO2 flow rate, but despite a higher bubble count than I have ever used, it never seemed to alleviate the issues.  It was because of this that I eventually removed this diffuser and went back to the older in-tank diffuser.  Right away, the tank started to bounce back with improved dissolved and dispersed CO2.

Final Thoughts on the In-Line CO2 Diffuser:

This product really has one weakness – the bubbles produced are not fine enough to be dispersed and dissolved efficiently into the water column.  That is kind-of a biggie as this is the entire purpose for a CO2 diffuser. If it had performance similar to the Bazooka in-tank diffuser, It would still be in our tank.  However, the priority for myself is in the plant growth, and that clearly suffers with this inline diffuser.

I understand one limitation in our application is the very short distance of outlet tubing after the device.  On a more traditional setup (such as installation after a canister filter) there would be more distance and this might improve the issue of dissolving CO2.

Inline CO2 Diffuser DIY project for Fluval Spec V Aquarium

Cost is also an issue.  The near $50 USD that we paid for this unit to be shipped to us is very high – about half of the cost of our Spec V.  Even if the performance was as desired, that is a large cost for only an aesthetic benefit. As I mentioned before, the exact same product is available by other vendors for about half the cost.

I hope that in the future this manufacturer improves the design of the ceramic to make finer bubbles, or another manufacturer steps in with a better performing in-line diffuser of similar size and form factor.

4 thoughts on “Inline CO2 Diffuser Review

  1. You installed the unit backward. The flow should be running in the other direction. Not sure if this would make a difference with the bubble size. I have one working on my aquascape. It is excellent. Bubbles are very fine…almost impossible to see unless my lights are on full intensity.

    1. The reality is that after the photos shown, I flipped the in-line diffuser. At any rate, orientation won’t matter. There is pressurized CO2 on one side, water on the other.

  2. I can second the previous comment. I have mine installed in a Coralife Biocube 14 and it produces very fine bubbles. I have a Vivid Creative Aquatics dual nozzle RFG return valve and I have to watch very closely to see the fine mist of bubbles. I’m at less than half a bubble per second so the rate is really slow and it randomly comes out one return nozzle or the other and also in random directions. When I turn off my pump and CO2, some gas collects in the top of the return pipe. When I start it back up, I always see a burst of bubbles come out.

  3. Apart from being installed backwards there should be a longer length of filter hose after the diffuser before the outlet. This allows the co2 to dissolve more before it enters the tank. They really are more suited to an external canister filter and a larger tank. For your size tank an internal co2 diffuser is more fit for purpose.

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