Using the canister filter as a co2 reactor - The Planted Tank Forum
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post #1 of 24 (permalink) Old 02-08-2020, 07:35 PM Thread Starter
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Using the canister filter as a co2 reactor

I have an inline diffuser and the instructions say that it should be attached to the canister outflow, so that CO2 is transferred directly into the aquarium. This is how I have had it setup for months now, so a question came in mind. Why don't we attach it to the inflow instead? The co2 would be forced to travel through all the filter media and sponge in the canister, which I presume would result in a very high dissolution rate like a reactor. What am I not considering?

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post #2 of 24 (permalink) Old 02-08-2020, 07:48 PM
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Apparently it's a bad idea because you increase pressure into the filter which shortens the life of the filter. Some people have mentioned explosive leaks. Not sure about that. But I do know it's noisy having any air in my filter. /Shrug
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post #3 of 24 (permalink) Old 02-08-2020, 08:01 PM
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Not a good idea because: 1) It can form air pocket and burb, 2) High CO2 and acidity can kill off bb in the canister media, and 3) Gas bubbles can cause cavitation, noise and erosion of the impeller.
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post #4 of 24 (permalink) Old 02-08-2020, 08:19 PM
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Why re-invent the wheel. We know it works at the outflow very well.


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post #5 of 24 (permalink) Old 02-08-2020, 11:10 PM
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Using the canister filter as co2 reactor is good idea. I have experience 5+ year's with dissolving co2 through canister filter. I'm using Fluval 406, Eheim 2075 and Sunsun 302 without any problem, no leaks, no problem with biological filtration, no problem with gasket wear off. Everything works fine. Only one thing, occasionally you can hear undissolved co2 going out through outlet, no big deal.
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post #6 of 24 (permalink) Old 02-09-2020, 02:41 AM
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In addition to some of the previously mentioned problems, you would want to use the outflow because it's already been filtered. That way you don't have to take the diffuser apart and clean it, or very rarely any way.
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post #7 of 24 (permalink) Old 02-09-2020, 02:52 AM
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Very old subject that has been debated many times with almost all suggesting that it just works much better on the output. Big reason for me not to even try is that it seems a really good way for any excess cO2 to collect at the top of the canister which is where the impeller sets. An impeller will not move air so I don't want it setting even partly in a bubble. Dry impellers wear very quickly as they normally use water as sort of lube.
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post #8 of 24 (permalink) Old 02-09-2020, 04:31 AM
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A different take....I've been injecting my CO2 directly into my canister filter from the time I began injecting CO2.....which has been close to (but not quite) 20 years. I'm still injecting CO2 into that same Ehiem canister filter I started with all these years later and it hasn"t missed a tick....still running on the original impeller although I have replaced the rubber seals a time or two. IMO, it's an effective and easy way to get that CO2 dissolved into the water.
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post #9 of 24 (permalink) Old 02-09-2020, 12:42 PM
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Originally Posted by jra2212 View Post
A different take....I've been injecting my CO2 directly into my canister filter from the time I began injecting CO2.....which has been close to (but not quite) 20 years. I'm still injecting CO2 into that same Ehiem canister filter I started with all these years later and it hasn"t missed a tick....still running on the original impeller although I have replaced the rubber seals a time or two. IMO, it's an effective and easy way to get that CO2 dissolved into the water.
The problem is there are many that do have problems depending on filter and levels of co2. I've never heard of anyone having issues at the outflow.
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post #10 of 24 (permalink) Old 02-09-2020, 07:48 PM Thread Starter
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Originally Posted by PlantedRich View Post
Very old subject that has been debated many times with almost all suggesting that it just works much better on the output. Big reason for me not to even try is that it seems a really good way for any excess cO2 to collect at the top of the canister which is where the impeller sets. An impeller will not move air so I don't want it setting even partly in a bubble. Dry impellers wear very quickly as they normally use water as sort of lube.
I figured I wasn't the first one to come up with this. That kinda makes sense, but I suppose it depends on the canister filter design a little. The newer eheim design seems to suck the water to the bottom and push it through the media, and what remains through the impeller. By my logic if the CO2 is ever going to dissolve, it would have dissolved by then. If it hasn't, it is possible the injection rate is too high. After googling one aquarist said that even if the pump ran empty the overheating is not as much of an issue with modern magnetic impellers. Some users said they had tried it, but none of them had measured the effects. I'm considering doing this with a bigger 400-500 liter tank I am planning in the future. For the 120 liter I have no I don't see much of a benefit with the exception of perhaps better visibility.

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Originally Posted by Kamilstrom View Post
Using the canister filter as co2 reactor is good idea. I have experience 5+ year's with dissolving co2 through canister filter. I'm using Fluval 406, Eheim 2075 and Sunsun 302 without any problem, no leaks, no problem with biological filtration, no problem with gasket wear off. Everything works fine. Only one thing, occasionally you can hear undissolved co2 going out through outlet, no big deal.
Thanks for sharing!

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Why re-invent the wheel. We know it works at the outflow very well.
In my opinion discussions related to possible improvements should be welcomed, not shut down. While something may work now does not mean it can't be improved upon.

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The problem is there are many that do have problems depending on filter and levels of co2. I've never heard of anyone having issues at the outflow.
The "problem" would be low dissolution an atomizer offers and not having to buy an actual reactor. The answer is basically the same as to why someone would want to buy a CO2 reactor. The biggest reason would be more efficient use of CO2 I guess, in the sense that you have to inject less for the same results and it is easier to disperse it around the tank with the water flow. Another one would be less microbubbles = more visibility. My tank for example has lights on an no CO2 injection only 1.5 hours per day.

So far I've found some users having issues with gas building up (which means injection rate was likely too high, and tuned for a setup where a lot of CO2 is lost), and others saying that it wasn't as efficient as they had hoped. Some people raised concerns about bacteria dying because of pH or high CO2 meaning low O2, but some users were quick to point out this not to be the case. If pH was an issue that would be a problem regardless of injection method (or just too much CO2), and CO2 levels are independent of O2 levels - it is possible to have both at high levels at the same time.

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post #11 of 24 (permalink) Old 02-09-2020, 08:04 PM
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In my opinion discussions related to possible improvements should be welcomed, not shut down. While something may work now does not mean it can't be improved upon.
Generally I agree with you, but using the intake of a canister filter for co2 dissolution has been discussed forever here. Just do a search. There is no benefit, but possible damage to filter. There are 3 main ways to diffuse co2, in-tank, in-line and reactor. They all work. Just look at different setups using all the methods. Look at aquascaping contest entries, etc. and successful tanks here. Dissolution percent accounts for pretty much nothing as we are talking pennies per month between the three methods for most tanks.

Look at these huge tanks at the ADA gallery using nothing more than an in-tank diffuser half way down.





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post #12 of 24 (permalink) Old 02-09-2020, 08:30 PM
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Bit tangential to the thread, but one advantage to the microbubbles vs. fully dissolved CO2 I've recently read about is that the former is more effective because the bubbles become trapped under the leaves, resulting in longer exposure and greater uptake. I can't comment on the validity myself, but perhaps others will.

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Wow, which plant is that? I've always wanted to do something like that with elatine triandra.
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post #13 of 24 (permalink) Old 02-09-2020, 08:30 PM
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Originally Posted by qoob View Post
Some people raised concerns about bacteria dying because of pH [...] If pH was an issue that would be a problem regardless of injection method (or just too much CO2)

No, because the pH in the canister would be much lower than the average pH in the whole system; the canister would be the area of lowest pH, by far. Whether that pH would be low enough to affect nitrifying bacteria (and whether that would even be noticed in a planted tank where much of the nitrification occurs in parts of the system other than the dedicated biomedia) is another question.
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post #14 of 24 (permalink) Old 02-10-2020, 06:22 AM
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Hi, I have a question. I setup a Serra reactor to my Fluval 306 canister filter. What would work best to distribute Co2?
1. Spray bar
2. Fluval output attachment ( that came with filter)
3. Lily pipes
Thank you for your help, much appreciated.


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post #15 of 24 (permalink) Old 02-10-2020, 08:40 AM Thread Starter
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Originally Posted by Asteroid View Post
Generally I agree with you, but using the intake of a canister filter for co2 dissolution has been discussed forever here. Just do a search. There is no benefit, but possible damage to filter. There are 3 main ways to diffuse co2, in-tank, in-line and reactor. They all work. Just look at different setups using all the methods. Look at aquascaping contest entries, etc. and successful tanks here. Dissolution percent accounts for pretty much nothing as we are talking pennies per month between the three methods for most tanks.

Look at these huge tanks at the ADA gallery using nothing more than an in-tank diffuser half way down.



I have done searches, the results of what I found are in the previous post. Sure ADA aquariums look great despite injecting with an in-tank diffuser, but you can be sure they have their flow set just right so that it works. Also the lights and ferts are tuned in perfectly for each tank. I've got Amanos life works in my book shelf and over 1985-2009 different tanks have tried quite different maintaining regimes. In my opinion 95% of why their tanks look like that is because of how carefully they are maintained. Then high quality substrate, lights, ferts and CO2 make sure there is growth to maintain. The same applies to contest tanks - you don't get high ranks in IAPLC unless you carefully maintain the scape constantly. And the picture that goes in the competition is often 1 out of multiple hundreds.

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Originally Posted by Socratic monologue View Post
No, because the pH in the canister would be much lower than the average pH in the whole system; the canister would be the area of lowest pH, by far. Whether that pH would be low enough to affect nitrifying bacteria (and whether that would even be noticed in a planted tank where much of the nitrification occurs in parts of the system other than the dedicated biomedia) is another question.
Why would the pH in the canister be lower? The water flows through at a rate that is like 150x the volume of the canister per hour. From my understanding what causes CO2 to decrease pH is the CO2 dissolving into the water and the carbonic acid lowering it. Once the CO2 has dissolved, it has turned into a liquid that flows out of the filter with the water. There isn't any more or less CO2 in the canister unless some of it gets stuck in its gaseous form and by then you are probably injecting too much anyway. As new CO2 enters the canister, water at the bottom of the tank (which arguably has the least amount of CO2) mixes in with it.

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Originally Posted by Rainer View Post
Bit tangential to the thread, but one advantage to the microbubbles vs. fully dissolved CO2 I've recently read about is that the former is more effective because the bubbles become trapped under the leaves, resulting in longer exposure and greater uptake. I can't comment on the validity myself, but perhaps others will.


Wow, which plant is that? I've always wanted to do something like that with elatine triandra.
I haven't yet seen any evidence on that aspect, but sounds plausible. Tom Barr (I think it was him) has linked to studies that prove plants can take in CO2 from microbubbles, so it does not have to be all dissolved to work. For me the main reason would be higher water clarity. The soda pop effect quite shocked me when I first started injecting CO2, and I presume people who are not used to it pay attention to the same thing. In my opinion the tank is at its most beautiful when lights are on and CO2 off, because of the bubbles.

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Originally Posted by kookyxogirl View Post
Hi, I have a question. I setup a Serra reactor to my Fluval 306 canister filter. What would work best to distribute Co2?
1. Spray bar
2. Fluval output attachment ( that came with filter)
3. Lily pipes
Thank you for your help, much appreciated.

Sent from my iPhone using Tapatalk
Whatever method can get you the best flow in all corners of the tank with plants is the best. I have a 3-part spraybar that allows me to direct the flow into 3 different directions. Lily pipes are just beautiful, don't know if they are the most efficient.
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Last edited by qoob; 02-10-2020 at 08:55 AM. Reason: new quote
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