# Silicone Tubing as CO2 Diffuser



## Hoppy (Dec 24, 2005)

Sometimes you can learn a lot by failing at a project!

I have been thinking that the reported high CO2 diffusion constant published for silicone tubing offers the possibility of using a long piece of that tubing, coiled in the substrate as a diffuser for CO2 that would release the CO2 where it would be most effective. So, I bought a 25 foot roll of Top Fin Silicone Airline Tubing, about $6 worth, and tested it today.

I made a bubble counter out of a club soda bottle to be sure it would be strong enough to withstand up to 40 psi. That was a very conventional bubble counter, with a couple of pieces of silicone tubing pulled through undersize holes in the cap, one deep into the bottle, the other ending near the top of the bottle.

Then I set up for a test, as shown:









I started with a little plastic check valve as a connector, but that leaked CO2 at the connections, so I replaced that with a short piece of rigid tubing:









I put the coil of tubing in a container with about a gallon of water in it, having to weight it down so it wouldn't float up. I purged the air out of the tubing by running CO2 through it for a minute or so, and clamped it shut by folding it over, tieing it with wire and adding a "bag clamp" to squeeze it even more.

Using the regulator, with the needle valve open more than normal, I raised the CO2 pressure to 5 psi, and got a surge of bubbles through the bubble counter, until the tube filled up to the 5 psi pressure. The final bubble rate was about 30 bubbles per minute.

Raising the pressure to 10 psi, 15 psi, 20 psi, 25 psi and 30 psi did nothing but reveal a tiny leak at the tubing connection going through the bubble counter cap - a very small leak, taking minutes to generate a fine foam of soap solution. The bubble rate decreased as the pressure went up until it was about 12 bubbles per minute at 30 psi.

Clearly, little or no CO2 was diffusing into the water. Certainly not enough was diffusing to be able to use the tubing for a CO2 diffuser. So, the idea isn't practical, if it would work at all.

This did prove that we can use cheap silicone air tubing for our CO2 systems, and not lose any CO2 as a result. The normal pressure our CO2 runs at, downstream of the needle valve is about 2 psi, so there should be no measurable leak at that pressure.


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## marrow (Feb 4, 2007)

Well done. Very clever demonstration and useful as well.


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## taekwondodo (Apr 16, 2006)

I had a big "discussion" with someone who used to sell CO2 equipment (and tubing) on this forum (and APC). Can't (cough-cough) recall who  - it was right after I highlighted the CO2/tubing loss being a MYTH.

I was pretty (what's the word) scolded that dare I say that vinyl or silicone tubing DID NOT leak CO2... We debated, and found a web-site that discussed CO2 diffusion between different membrane types - and it provided a formula to indicate what that loss through the membrane would be, based on the difference in pressure inside the tubing vs. outside the tubing.

After doing (and posting) the math, I figured I'm losing less than a nickle's worth (yes, $0.05) of CO2 out of a 20lb tank. 

Then the "discussion" got some "clarification" - (cough, cough, cough) - "Well, that's because you don't have any pressure in your tubing..."

Don't most of us run fairly low-pressure (mine's actually slightly negative - as I feed a mini-venturi).

I didn't want to do the math again...

Looks like your experiment is THE Myth-buster 

Thanks!

- Jeff

p.s., we should start a "Common Aquarium Myths" series of threads...
p.p.s, LOL - I got a ghetto counter just like that one


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## CL (Mar 13, 2008)

Ha, good news for me. I'm glad that I never actually got around to paying for that "co2 proof tubing"


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## taekwondodo (Apr 16, 2006)

Seriously though - the tubing "co-efficient" for osmotic transfer for the "CO2-proof" tubing is like a factor of 1000 over that of Vinyl (which most people keep confusing with Silicone tubing). But again, it's all based on the differential pressure.

Maybe later I'll look up the thread (Can't remember if it was here or APC) and post the table/math.

- Jeff


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## eyebeatbadgers (Aug 6, 2007)

Good results hoppy. Even if there is some difference in tubing, does it really matter? Co2 is cheap gas!


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## Hoppy (Dec 24, 2005)

Unlike those who use silicone tubing for their CO2 systems I was hoping for the tubing to be very leaky. I was hoping, based on a calculation I made, that the tubing would leak CO2 at about a 10 bubbles per second rate, with 25 feet of it, and 30-40 psi pressure. So, for my purposes this was an utter failure. All I can conclude about the tables of "leakiness" for various material tubing for CO2 is that the units used for the coefficient are a hodgepodge of inconsistent units - like mm for thickness, meters for length, cm for diameter, inches of mercury for pressure, etc. That could cause several orders of magnitude errors in calculating the "leakage" unless you guessed the correct units. I must not have guessed correctly. Now I'm simmering the idea of using a RFUG of some sort - so far it hasn't cooked.:redface:


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## taekwondodo (Apr 16, 2006)

Hoppy said:


> So, for my purposes this was an utter failure.


Funny, Columbus was disappointed with his discovery of the Americas...

As long as we can dispel an Aquarium myth that people waste lots of money on, then success be damned!

And as Edison replied if he was discouraged with the number of failures he experienced when he tried 1000 times to make a lightbulb, he replied, "I didn't fail 1000 times, I learned 1000 ways how NOT to make a lightbulb." (or something like that). 

- Jeff


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## fishyface (Oct 7, 2004)

isn't regular airline "supposed" to break down over the long run. i thought that over time it became brittle leading to small leaks, i don't believe it would happen immediately if it was going to happen at all.


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## Left C (Nov 15, 2003)

Hi Hoppy

Would it be possible that the CO2 bubbles may be dissolving in the water so fast that you can't see the bubbles form?

If you could accurately weigh the container of water before and after the addition of CO2, would you possibly see an increase in the weight? CO2 is ~ 37 times more soluble in water than O2 is?

*Solubility of CO2 and O2 in H2O*

*- CO2*

0.1449g in 100g H2O @ 25° C or 77° F

0.001449 parts or 1449 ppm

*- O2*

0.003931g in 100g H2O @ 25° C or 77° F

0.00003931 parts or 39.31 ppm



1449 ppm / 39.31 ppm = 36.86084966

CO2 is 36.86084966 or ~ 37 times more soluble in H2O than O2 is.



Left C


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## Hoppy (Dec 24, 2005)

LeftC, I wasn't looking for bubbles in the one gallon of water the tubing was in, just measuring the CO2 flow rate in bubbles per second in the bubble counter. I doubt ever seeing bubbles from the CO2 leakage out of silicone tubing, because that CO2 would almost certainly just dissolve as it comes out.

Fishyface, regular vinyl airline tubing does get brittle with even air use, but more so with CO2 use. But, the silicone tubing I have used doesn't do anything but get slightly more stiff, and that takes quite a few months to notice at all. The table of diffusion constants for various tubing materials shows silicone tubing to be by far the worst material to use for CO2. That is what I was testing


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## Green Leaf Aquariums (Feb 26, 2007)

Cool test Hoppy 

Sure you can use tubing X to get co2 from a-b. But in due time your tubing will harden then split or crack. By the time this happens you will probably have long forgotten that you used tubing X. And your new leak has been born.
Is it worth the chance of having your tubing harden and crack? Not for me, but maybe for some folks.

Cheers, Orlando


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## Hoppy (Dec 24, 2005)

Orlando said:


> Cool test Hoppy
> 
> Sure you can use tubing X to get co2 from a-b. But in due time your tubing will harden then split or crack. By the time this happens you will probably have long forgotten that you used tubing X. And your new leak has been born.
> Is it worth the chance of having your tubing harden and crack? Not for me, but maybe for some folks.
> ...


I would never claim that my test shows that this silicone tubing is as good as the CO2 proof tubing made for that purpose. What it does is show that the worries about losing half the CO2 before it even gets to the tank are very much over done. I continue to use CO2 resistant tubing on my CO2 system, except for short pieces, usually in the tank.

Has anyone actually experienced silicone tubing hardening and cracking as you predicted?


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## Green Leaf Aquariums (Feb 26, 2007)

The tubing will usually harden at the connection points only, not the length of the tubing.
I would have to cut the ends off to make for fresh malleable tubing every time I cleaned a diffuser. The tubing would turn into a hard plastic almost.
But it still did the job no doubt, until I started using Tygon tubing it was smooth sailing.

I wonder what it would take to diffuse co2 in the substrate and if it would be beneficial? Have you thought of trying anything else for this Hoppy?
It sure sounds interesting.

Regards, O


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## Ryzilla (Oct 29, 2005)

Orlando said:


> The tubing will usually harden at the connection points only, not the length of the tubing.
> I would have to cut the ends off to make for fresh malleable tubing every time I cleaned a diffuser. The tubing would turn into a hard plastic almost.
> But it still did the job no doubt, until I started using Tygon tubing it was smooth sailing.
> 
> ...


I concur with this observation. It is not unusual for me to have to clip off the end, maybe 1/2" of silicon tubing every now and then when I used it with my DIY CO2 set up. I also never noticed the hardening in the middle portions of the tube. 

If so much CO2 was lost in silicon tubing then the water you have the tubing in should have a lower ph than the tap where you took the water from. Great experiement:thumbsup:


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## snafu (Oct 9, 2004)

the calculations for diffusion rates are usually done using air at standard atmosphere and pressure. recall that the rate of diffusion depends on the difference in partial pressures of CO2. i suspect immersing the tubing in water changes things a bit.


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## mistergreen (Dec 9, 2006)

I remember seeing somebody used part of a breather bag to make a drop checker membrane. Maybe it can work the other way and become a diffusor.


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## taekwondodo (Apr 16, 2006)

Ryzilla said:


> I concur with this observation. It is not unusual for me to have to clip off the end, maybe 1/2" of silicon tubing every now and then when I used it with my DIY CO2 set up. I also never noticed the hardening in the middle portions of the tube.
> 
> If so much CO2 was lost in silicon tubing then the water you have the tubing in should have a lower ph than the tap where you took the water from. Great experiement:thumbsup:



Are we sure we are talking about SILICONE tubing - the clear stuff we get at the store is really a clear vinyl tubing (most people call it silicone because it is clear), and gets hard after a long period of use - this stuff here:

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=6124

Which is almost as good as Tygon (or, according to this, as good or better)...

Real silicone tubing stays flexible for just about forever.


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## EdTheEdge (Jan 25, 2007)

Bravo Zulu Hoppy! You da man! I have long suspected the urban myth about using silicon tubing with CO2.


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## imeridian (Jan 19, 2007)

taekwondodo said:


> Are we sure we are talking about SILICONE tubing
> --snip--
> Real silicone tubing stays flexible for just about forever.



I can verify/confirm what others are saying here. I have had blue silicone tubing stiffen from exposure to CO2 and water. The tubing outside of the water was fine, but anything in the water hardened.


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## plantbrain (Dec 15, 2003)

taekwondodo said:


> I had a big "discussion" with someone who used to sell CO2 equipment (and tubing) on this forum (and APC). Can't (cough-cough) recall who  - it was right after I highlighted the CO2/tubing loss being a MYTH.
> 
> I was pretty (what's the word) scolded that dare I say that vinyl or silicone tubing DID NOT leak CO2... We debated, and found a web-site that discussed CO2 diffusion between different membrane types - and it provided a formula to indicate what that loss through the membrane would be, based on the difference in pressure inside the tubing vs. outside the tubing.
> 
> ...


Well, it could not had been me, I do not post on APC:redface:
Nor would I ignore practical evidence to the contrary:icon_roll
I've used silicone tubing for many years in large manifolds.

Still, I argued with Vaughn that the rate would be much too low to be of any use, this seems to confirm it, but tinkering around, you often find other things that prove or falsify something. But if you look at those cheesy membrane style CO2 diffusers they sell, AM makes one.........they do not do much.
They have little control, etc. Vaughn is aware of all that. That was not what interested him later.

The other thing is that Bubble counter he has, that thin bottle etc might not hold gas well right? What are they designed to hold in the first place with logn shelf lifes and under high ppms of CO2? :redface:

Somehow that CO2 is fine in there, but not under a lot of psi's...........

Snafu also brings up the PARTIAL pressure in water vs the air. This will reduce the transfer rate(harder to go across that layer, diffusion boundary vs in air).
So now Vaughn will start to play with atomized CO2 an a RFUG(as History once again repeats itself).

If you search my name and RFUG, you will see all sorts of post from days gone by. I used them to apply higher flow rates and lower ones, I could control them using flow rates from a pump, into the gravel, much like a heater cable theory that assumed that increased flow brought more O2 to the sediment which helped roots......

This is curiously the exact same arguement used with ADA powersand proponents to justify their purchases:icon_roll

So based on that, I should have better results with than without, however, th never happened, then these heat cable folks claimed it was too fast of flow:icon_roll
But it's easy to change the rate of flow, they never once have ever given me a rate of flow that is "optimal".

Never, not once, not a redox level, not an O2 level that's optimal, nothing.
I, Claus of Tropica, and others have and it's the same as no cables, no added material for 2-3 mm grain sizes(ADA aqua soil tends to be even larger, but the powder is much smaller- so there's conflict in their logic- cap it with powder, but then add more O2 with large pumic? That makes no sense, they nullify the other).

About 1/2 liter per meter ^2 per day is optimal for plant roots, which is about normal diffusion, like they have in natural systems. Go figure.

RFUG's can be manipulated very well and are cheap(about 10-12$ for a typical set up), easy to make, much better than UG filter plates, less prone to burping and allow the entire depth of the gravel to be used. They do not fair well with organic macro laden sediments(messier).

They do keep the gravel clean and you can prefilter the water, add heated water, add CO2 etc.

CO2 mist/bubbles bubble up, if they do not get dissolved, over the entire bottom of the gravel hitting the bottom of leaves.

This is perhaps the best point of entry for CO2 in planted aquariums.
Hard to argue with Vaughn's notion about adding CO2 as above, but he just needed a better method to control the rates of CO2.

Still, I didn't fail 1000 times, I learned 1000 ways how NOT to add CO2.
Each time Vaughn, myself, etc, ruled out one possibility.

There's is a lot to be said for that, I ruled out using heating cables, ruled out not fearing PO4 or adding it, ranges of CO2, NO3, Toxicity, high K+ issues, algae etc......... Then I'm one step closer to figuring out how to do it "best", and what the trade offs are along the way for each method.

Regards, 
Tom Barr


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## Hoppy (Dec 24, 2005)

Tom, last night I resurrected your old posts in APD (The Krib) about RFUGs. Very interesting! Back in '99 you were pretty enthusiastic about them. But, nothing that I read there mentioned CO2 and RFUGs. 

The thing driving my interest in this is my disgust with my current CO2 injection methods, which never seem to quite be adequate, and which always fail as the plant mass gets bigger. I figure the best possible place to introduce the CO2 would have to be right under the plants at substrate level and uniformly all over the substrate. Silicone tubing would have been such a neat way to do that.:icon_sad:

I'm thinking about an external CO2 reactor in the filter output line, with that going into a RFUG. Possible problems with that seem endless at the moment.

I should be honest here: I'm a retired engineer and what I miss most about not working is working on problems like this. I get much more fun doing this than raising plants, and even more fun than keeping fish. To each his own!


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## snafu (Oct 9, 2004)

EdTheEdge said:


> Bravo Zulu Hoppy! You da man! I have long suspected the urban myth about using silicon tubing with CO2.


i'm not sure this proves anything? it's well known that silicone tubing has a high permeability coefficient for CO2. it's not hearsay or some myth. companies who make tubing actually do test tubing for permeability for different gas types usually under more rigorous testing. that being said, not all silicone tubing is the same.

in most cases, it's not something you have to worry about, esp if you are running CO2 lines into a reactor, venturi, bubble ladder, or some other lower pressure application. even if you aren't, does it matter anyways? CO2 is cheap. the places most prone to losing CO2 are the interconnects, and that's where other thicker tubing shines. 

the rate of loss depends on the area of the membrane (i.e., length and inner diameter of the tubing), so it's good practice to keep tubing lengths to a minimum. if you are running 20 ft of tubing into a diffuser, then CO2 loss could be an issue. a better practical test, would be to run different lengths and types of tubing into a diffuser while maintaining CO2 or pH levels and then determine how much CO2 was required over some time period. of course, that would be a PITA to do.

---

i thought i'd add that i use silicone tubing. heh


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## taekwondodo (Apr 16, 2006)

plantbrain said:


> Well, it could not had been me, I do not post on APC:redface:
> Nor would I ignore practical evidence to the contrary:icon_roll
> I've used silicone tubing for many years in large manifolds.


Nope -wasn't you (and you hadn't left, or had just left APC at the time) - we've "discussed" other things... . I failed continually at EI  - I must be in the rare minority... (Next time you're planning on driving through Gilroy, feel free to ping me and stop by - I'll spark up the Grill and throw some dead-cow on it in trade for pointers and personal review )...

This was with someone blabbing that they lose 6% of their CO2 per-foot (based on equation I think Snafu arrived at) - well, this was at extremely high differential pressures between the interior/exterior of the Silicone tubing. Oh - btw, this person also sold regulators... and coincidentally "CO2-proof" tubing :eek5:

When you do the REAL math, considering that 95% of us have a "final stage" regulator with a needle valve, and don't use high-pressure beyond the needle valve - it's literraly about $0.05 per 10lb bottle that's lost (because there's almost no differential between the inside/outside of the tube... certainly not more than a fraction of a PSI...).

But Hoppy's experiment is interesting and makes one wonder if this "transmission" coefficient (there's a link on PTDN somewhere) is accurate - if he's applying any real significant pressure, he should be seeing something (and isn't).

- Jeff


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## taekwondodo (Apr 16, 2006)

imeridian said:


> I can verify/confirm what others are saying here. I have had blue silicone tubing stiffen from exposure to CO2 and water. The tubing outside of the water was fine, but anything in the water hardened.


Odd - I have clear Silicone (not Vinyl), and been using it for well over two years. It's not hard at all - anywhere along the tubing. And mine feeds into the input of my closed loop (at the water line). It feels a little goopy (you made me go run and look/feel it when I saw your response...).

I can't imagine real silicone doing that. This stuff (Silicone), in addition to holding 95% of our tanks together - staying somewhat spongy for decades, also is used as 20-year caulking around homes (not the acrylic stuff, that hardens in 3-4 years, but real silicone caulking) and takes a severe environmental beating (UV, big temp changes, etc...).

The Vinyl stuff I've used gets hard within a year (whether I'm using CO2, or Air).

- Jeff


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## snafu (Oct 9, 2004)

the calcs i've seen done were not for super high pressure but more in the range of 15psi. the important thing to note is that when you compute the partial pressure gradient, one side has pure CO2 while the other has the partial pressure of CO2 in air (much lower). i think the mole fraction of CO2 in air is like 3%. so basically, even though the pressure in and out may be similar, there can still be a significant gradient.


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## Hoppy (Dec 24, 2005)

But, any CO2 that escapes the tubing due to anything from leaks to absorption to diffusion through the tubing, has to be replaced by more CO2 to keep the pressure the same in the tubing. That is why a bubble counter located at the beginning of the run of tubing has to show a bubble rate that is at least as great as the amount being lost. And, my bubble counter showed only a very small bubble rate, not much greater than the observable leakage I had.

I have no doubt that some CO2 does escape through silicone tubing. My conclusion from that admittedly crude experiment is that the magnitude of the loss of CO2 is far less than one should expect if all of the talk about how bad silicone tubing is for a CO2 system were correct. And, now that I think about it, given the tiny loss I was seeing, the other tubing materials are extraordinarily leak free, since their "gas transmission" factor is at least 1000 times less than that for silicone tubing. Again, now that my brain is in gear, there is another rubber material that is gas permeable to the extent that it can't be used for bladder type hydraulic accumulators - ethylene propylene rubber. I will do some research on that too.


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## EdTheEdge (Jan 25, 2007)

snafu said:


> i'm not sure this proves anything? it's well known that silicone tubing has a high permeability coefficient for CO2. it's not hearsay or some myth. companies who make tubing actually do test tubing for permeability for different gas types usually under more rigorous testing. that being said, not all silicone tubing is the same.
> 
> in most cases, it's not something you have to worry about, esp if you are running CO2 lines into a reactor, venturi, bubble ladder, or some other lower pressure application. even if you aren't, does it matter anyways? CO2 is cheap. the places most prone to losing CO2 are the interconnects, and that's where other thicker tubing shines.
> 
> ...


I agree that this is not something that I loose any sleep about. When a 10LB tank lasts me anywhere between 6 to 9 months or more I am satisfied.

I gave Hoppy accolades not because I feel theat this is the end all experiment but because I have not seen anyone testing the opposite. Indeed my own useage of Silicon tubing in some of my tanks does not prove the CO2 loss that I have read about many many times. I have found that my CO2 refills last just as long with silicon tubing as with my CO2 resistant tubing.

I would be interested in seeing someone actually test this so called loss of CO2 while using silicone tubing.


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## Hoppy (Dec 24, 2005)

EdTheEdge said:


> I would be interested in seeing someone actually test this so called loss of CO2 while using silicone tubing.


It is a very hard test to do, primarily because the loss of CO2 is so small you have to have absolutely no leakage anywhere else to make the measurement mean anything. If I really, really felt it was important to find the answer with some accuracy I would test at least 100 feet of tubing at once, both in air, then in water. I think I would do the work required to make a bubble counter that couldn't leak at all - not that I have any idea yet how to do that - then do very much what I did this time, possibly using a more accurate pressure gage. And, if someone else were paying for it, I would use a pressure transducer recording the pressure vs time in the tubing being tested. Also, I would take great pains to thoroughly flush out all air from the tubing, so the pressure gage was indicating CO2 partial pressure.


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## airborne_r6 (May 2, 2008)

plantbrain said:


> ...the bottom of leaves.
> 
> This is perhaps the best point of entry for CO2 in planted aquariums.
> ...


Are aquatic plants the same as terrestial plants in that CO2 and O2 primarily enter and exit the plant through the stomata on the underside of the leaves?


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## Left C (Nov 15, 2003)

My first planted aquarium had 2 fluorescent bulbs, Flourite, a Penn-Plex UGF with a Penguin 660R power head for the RFUGF. It kept the substrate clean with an auxiliary filter.

I wonder if feeding the CO2 into the 660R to blow CO2 through the substrate work. It would be picking up some of the ferts from the water column and they would then go through the substrate too. The MTS may not like this though.

Penn-Plex UGF and Penguin 660R power head.


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## Hoppy (Dec 24, 2005)

I haven't been able to find much of anything about results of pumping CO2 enriched water through a RFUG. Several write-ups mention that a person is going to try it, but never a followup about how it worked out. Intuitively I think it has to work well, but my intuition can be 180 degrees out of phase with reality.

I can't see a problem with pumping fertilized water through the substrate. If the substrate has a good CEC I can even see this being beneficial, because it would act much like a fertile substrate (ADA Aquasoil), with the ferts being constantly replenished. I'm going to have to rebuild my tank soon in any case (I have to because I want to), so this seems like an interesting way to go.

The argument that you can't clean any undergravel filter without completely disassembling the setup doesn't bother me at all. I don't think I would ever go much longer than a couple of years between doing so anyway.

When I start this "project" I will start another thread about it - who would have imagined ever seeing another thread on a DIY RFUG setup, or any UGF setup, for that matter? It will be like a trip to the past!


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