CO2 diffuser (for nano tanks)

[Marcel G]

Inspired by @Yugang's "CO2 spray bar" I'm posting a brief tutorial on making a simple "CO2 diffuser" (aka "CO2 bell") suitable especially for nano aquariums:

Probably the easiest and cheapest way to ensure a stable CO2 concentration in the aquarium is to place a glass beaker (bottom up) of a calculated size in the aquarium, into which we will supply concentrated carbon dioxide from a cylinder (see picture below).



Important: It is important that (1) the beaker is filled with gas to the brim, (2) the hose supplying CO2 to the beaker is inserted inside it, (3) the neck of the beaker is the correct size (= diameter), (4) the water, which is in contact with the carbon dioxide inside the beaker, is flowing, and (5) the beaker is placed as straight as possible.

(1) CO2 supply in slight excess

The first condition is achieved by supplying a little more gas into the beaker than will dissolve spontaneously into the water, with the excess gas escaping through the spout (overflow) away to the surface.

(2) Hose ends inside the beaker

Even if it doesn't seem like it, it is not only CO2 from the tubing that will get into the gas in the beaker, but also various other gases from the aquarium water (e.g. oxygen or nitrogen). This will cause the concentrated carbon dioxide in the beaker to dilute slightly over time. By inserting the tubing into the beaker, we ensure that the new gas that is fed into the beaker is dissolved without any residue, so that only the old (slightly diluted) gas escapes through the overflow. If we were to place the end of the tubing under the beaker, the bubbles escaping from the tubing into the beaker might not get into the beaker when they hit the gas surface in the beaker, but might slide along the surface to the spout (overflow) where they could escape to the water surface. Although this problem is likely to occur only in very small containers, it is better to try to prevent it.

(3) Correct diameter of the beaker

As for the required diameter of the beaker, we calculate this using the following formula:

⌀ = 2 * √ ( surface / 17.7 * coeffient / π )

• surface = the surface area of our aquarium (in mm2) ← see the area highlighted in blue in the picture
  • length and width of my aquarium (height is not important here): 300 x 250 mm
  • water surface area = 300 * 250 mm = 75,000 mm2
• coefficient
  • for a pH drop of 1.5 (~60 ppm CO2) = 1
  • for a pH drop of 1.2 (~30 ppm CO2) = 0.5
  • for a pH drop of 0.9 (~15 ppm CO2) = 0.25 ← the preferred value in my tanks

Example of calculation:

⌀ = 2 * √ ( 75,000 / 17.7 * 0.25 / 3.14 ) = 37 mm

Result: To achieve a concentration of ~15 ppm CO2 in my tank you need a beaker with an inner diameter of 37 mm (or a container of any other shape with a surface area of ~1060 mm2 => 75,000 / 17.7 * 0.25).

(4) Suitable water flow

A standard aquarium filter or a jet pump is sufficient to ensure a suitable water flow.

(5) Suitable fixing of the beaker

Although not the most aesthetically pleasing solution, the easiest way of attaching the beaker to the aquarium wall seems to me to be by means of two zip ties with suction cups [used to hold the glass thermometer]. Someone may come up with a better solution.



The principle of this method consists in the spontaneous diffusion of concentrated carbon dioxide from the beaker into the surrounding water until an equilibrium is established between (1) the rate of dissolution of CO2 in the water, (2) the rate of its degassing and (3) the rate of its consumption by plants. The genius of this method is that with a properly sized beaker, only the required amount of CO2 will dissolve in the water, which will be absolutely constant.

Note: Different sized aquariums require different sized beakers to achieve the same CO2 level.

 

[Yugang]

Thank you for your really nice post @Marcel G , and indeed I hope that more users will recognise the value for small tanks as compared to diffusers.

coefficient

• for a pH drop of 1.5 = 1
• for a pH drop of 1.2 = 0.5
• for ~15 ppm CO2 = 0.25 ← the preferred value in my tanks

In our private communications I estimated the 0.25 factor for 15 ppm, as this is what I have been using for the past 2 months on my own tank. Mathematically this may not be entirely correct, and if anyone interested in a more solid estimations I can calculate a more precise value later.

 

[Marcel G]

In our private communications I estimated the 0.25 factor for 15 ppm, as this is what I have been using for the past 2 months on my own tank. Mathematically this may not be entirely correct, and if anyone interested in a more solid estimations I can calculate a more precise value later.

I would certainly be interested in more detailed calculations. If you could upload an excel file (?) here, that would be great.

PS: I don't even know how many ppm the 1.5 or 1.2 unit drop in pH actually corresponds to. And for the third option, you mentioned that it corresponds to about 15 ppm CO2, but again I don't know how much of a pH drop that corresponds to. So any clarification would be appreciated.

I tested two sizes of containers (or "cups/beakers") in combination with a drop checker. The first one with a diameter of 70 mm resulted in too much CO2 (yellow dropchecker). The second one with a diameter of about 35 mm (the 50 ml beaker in the photo) resulted in less CO2 (dark green drop checker). I'm just guessing the CO2 concentration from this, but it could be around the 15 ppm mark, which is exactly my target.

 

[Yugang]

I would certainly be interested in more detailed calculations. If you could upload an excel file (?) here, that would be great.

PS: I don't even know how many ppm the 1.5 or 1.2 unit drop in pH actually corresponds to. And for the third option, you mentioned that it corresponds to about 15 ppm CO2, but again I don't know how much of a pH drop that corresponds to. So any clarification would be appreciated.

I tested two sizes of containers (or "cups/beakers") in combination with a drop checker. The first one with a diameter of 70 mm resulted in too much CO2 (yellow dropchecker). The second one with a diameter of about 35 mm (the 50 ml beaker in the photo) resulted in less CO2 (dark green drop checker). I'm just guessing the CO2 concentration from this, but it could be around the 15 ppm mark, which is exactly my target.

To be honest with you @Marcel G I am also not entirely clear, and would need more time to think this through and validate with measurements.

What we understand quite well, is that with applying the 17.7 ratio we achieve around 1.5 pH drop. This has been confirmed with pH probes, on multiple tanks. I am sure you are also aware that we have a logarithmic relationship between CO2 ppm and pH, so that a 0.3 difference in pH drop corresponds to 50% less CO2 ppm. Therefore, if we inject 50% less with a 50% smaller reactor, this balances with a 50% less outgassing via the water surface (plant consumption is much less than this at these levels), and therefore very likely a 1.5 pH drop would be reduced to a 1.2 pH drop. And then an 0.9 pH drop would again be 50% less, ie with a 4 times smaller reactor than what we started with. So far so good.

I am always cautious to call a CO2 ppm, as it is really easy to make mistakes here. I also don't care too much, as what really counts is stability and this can be verified by seeing the stability of the pH reading. I have my injection so well under control that I decided not to purchase a new pH meter, when my old one broke down after several years of intensive use. I just use a drop checker to make sure I am in the right ball park, and then with overflow mode keep it stable at whatever ppm that is.

I am currently using a 2 dKH drop checker, and see the color with still some hints of blue, not entirely lime green. So this is what I estimate around 15 ppm, or perhaps a bit less. But, I can't verify with a pH probe, and as explained I am not really interested in that either.

I hesitate to give an answer right now how to calculate the CO2 Spray Bar for 15 ppm as I worry that I could make mistakes with my extrapolations and assumptions, and cannot double check with a pH probe or professional CO2 probe. I also can't translate directly from my own tank, as I have a lid covering it as part of another experiment, that reduces CO2 consumption.

So please bear with me, but I will definitely post when I have new insights and am confident enough that these are correct. I believe the best you can do is using a 2 dKH drop checker, and experiment a bit with the correct size of your cup / CO2 Spray bar until you see it lime green.

Just as a reminder, for correct operation in overflow mode you want some water flow under the reactor and with that avoid a diffusion limited stagnant boundary layer.

 

[Marcel G]

To be honest with you @Marcel G I am also not entirely clear, and would need more time to think this through and validate with measurements. What we understand quite well, is that with applying the 17.7 ratio we achieve around 1.5 pH drop. This has been confirmed with pH probes, on multiple tanks. I am sure you are also aware that we have a logarithmic relationship between CO2 ppm and pH, so that a 0.3 difference in pH drop corresponds to 50% less CO2 ppm. Therefore, if we inject 50% less with a 50% smaller reactor, this balances with a 50% less outgassing via the water surface (plant consumption is much less than this at these levels), and therefore very likely a 1.5 pH drop would be reduced to a 1.2 pH drop. And then an 0.9 pH drop would again be 50% less, ie with a 4 times smaller reactor than what we started with. So far so good ...

Thanks a lot for the explanation. I thought you had a formula for that somewhere, you just have to find it. If it requires some complicated measurements and verifications, just leave it out. The important thing is that it can be verified with a drop checker.

One more time: Thanks a lot! I personally found it very helpful.

 

[Marwen]

Interesting! Thanks for sharing Marcel.

It would be nice if some manufacturers made a product using this concept that would be as visually clean and minimalistic as possible.

1. This could be something like a glass box inside the tank (like the becker here). The only disadvantage with this is the space it takes.
   
   
2. Or it could be something that could be hung outside of the tank similar to a hang-on-the-back filter. It would be powered by its own internal pump so the water flow rate would be a testable and known variable in this case. When gas fills the chamber beyond a certain level the overflowing excess would be purged out back into the tank (and off into the air) by the pump similar to how the overflow mode works in a Yugang reactor.
   
   
3. Or it could be a stage inside a HOB filter after the mechanical/biological filtration stages. But here, the filter clogging would impact flow rate and we'd lose having that as a known variable.

It would make it possible to sell as a mini reactor for nano tanks where the CO2 dissolution can be well estimated per tank size.

Since I dislike having a diffuser in my in my 30L nano tank, maybe I should order some acrylic sheets and tubes and a small self-priming pump and test out idea #2 above.

 

[Koan]

that would be as visually clean and minimalistic as possible

Hmmm..

"Koan diffuser" has a nice ring to it..

 

[gjcarew]

Why does the beaker have to be filled to the rim? I would assume for a straight-sided beaker, the gas exchange would be the same regardless of how "full" of CO2 the beaker is, since the cross-sectional area of the gas-water interface is the same at all heights of the beaker

 

[Yugang]

Why does the beaker have to be filled to the rim? I would assume for a straight-sided beaker, the gas exchange would be the same regardless of how "full" of CO2 the beaker is, since the cross-sectional area of the gas-water interface is the same at all heights of the beaker

I hope I understand your question correct.
You want some CO2 to overflow, so that we stabilise the injection rate into the tank water. When there is no CO2 overflow the injection will still work, as any 'conventional' reactor, but tank CO2 ppm stability will depend on the stability of the CO2 regulator or CO2/pH controller.