# Passive diffusion=best diffusion?



## aquanauts (Jan 18, 2015)

When most people think of co2 diffusion, they think of ceramic diffusers or bubbles being chopped by an impeller of a pump producing micro-bubbles, some of which tend to reach the surface. I have always been told that the key to co2 diffusion is contact time with co2 and the water itself. So, that is when passive diffusion comes to mind, where you basically fill an upside down container with co2 under water (like a co2 bubble constantly touching the water). I have always thought this was the best method for co2 diffusion, but others say reactors or ceramic dffusers are better. Does passive co2 diffusion work? Is it truly the supreme method of diffusion?


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## evil nick (Oct 20, 2014)

I've often thought about putting some upside down cup on a suction cup above my diffusers to stop the bubbles in place and see if they pop in the water before rising out. 
I'm lazy at all the wrong moments though


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## dukydaf (Dec 27, 2004)

CO2 diffusion = contact time+ contact surface area

Many and small CO2 bubbles can have a larger surface area in contact with aquarium water than the recipent you describe, therefore CO2 would be diffused faster. If you feel the diffuser does not give you sufficient CO2 you can certainly add a 'cup' that catches those bubbles. The problem here is that you want good flow of water under that cup.

Reactors mainly focus on indefinite contact time while seeking to enhance surface area by complicating the water path inside the reactor ( sponges and other). Ceramic diffusers seek to make bubbles as fine as possible (contact surface) and the bubbles should be swept away by the water current (increased contact time)

PS: I hate to label anything as 'thebest'. The correct answer is ' It depends'


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## evil nick (Oct 20, 2014)

I leave my diffuser 






almost under my whisper filter so the bubbles get pushed back down when they rise. I can actually see them being swept around. I tend to like the look of micro bubbles even though everyone else hates them. 
This diffuser works relatively well at making very fine bubbles.


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## essabee (Oct 7, 2006)

aquanauts said:


> When most people think of co2 diffusion, they think of ceramic diffusers or bubbles being chopped by an impeller of a pump producing micro-bubbles, some of which tend to reach the surface. I have always been told that the key to co2 diffusion is contact time with co2 and the water itself. So, that is when passive diffusion comes to mind, where you basically fill an upside down container with co2 under water (like a co2 bubble constantly touching the water). I have always thought this was the best method for co2 diffusion, but others say reactors or ceramic dffusers are better. Does passive co2 diffusion work? Is it truly the supreme method of diffusion?


I would not hold with the statement that chopping the CO2 into bubbles is active diffusion. It may increase the surface area in contact with water but the diffusion rate per unit surface area remains the same otherwise as between a nearly pure CO2 atmosphere and water.

"Active diffusion" should be some process by which the rate of diffusion per unit area of contact of CO2 gas and water, is enhanced. The easiest way of enhancing this diffusion rate is having the CO2 gas at a higher pressure than the atmosphere while in contact with water - yes that will cause "Active diffusion" and super saturation of the gas in water will occur. In nature we do find water super saturated with gasses - it is a common phenomenon down stream from water falls, white-water, hydel generators, geysers and gushers etc.

Such "Active diffusion" is certainly impossible when the CO2 gas is released through ceramic diffusers into the tank as these bubbles are only so much higher than the atmospheric pressure as the additional water pressure at point of release in the tank - a depth of say a maximum 2' and rapidly diminishing.

In reactors on the other hand "Active diffusion" would and does occur. Even in a simple Rex Grigg type reactor the water pressure is much higher than the tank depth and the there has to be a correspondingly higher pressure for the CO2 gas in it. As these reactors are inline with water flows it has many similarity with water-falls and white-water streams - where nature creates gaseous super solutions. Further more some of these reactors use choppers and impellers which further emulates the conditions of white-water streams. So it is not surprising that these reactors do "Active diffusion" and create water super saturated with CO2.

We do need this CO2 super-saturated water to enhance the CO2 level of the tank - that was the reason in the first place to have these inline gadgets. The inline reactors and diffusers do this much more efficiently than those in-tank ceramic and other diffusers.

So from the point of view of the needs of our CO2 enhanced tanks - I say that these "Active diffusion" systems are better. Then its only my personal opinion.


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## aquanauts (Jan 18, 2015)

Thanks essabee! That was so helpful!

Sent from my SM-G920P using Tapatalk


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## robbhammack (Nov 26, 2015)

way back in the dawn of time, before CO2 accessories became available at a price for mere mortals, I ran a DIY yeast system with a homemade diffuser that worked as well as anything I've seen lately. 
It worked like so: co2 tube feeding into a powerhead intake, short hose on powerhead output to barb fitting on a hose adaper that was siliconed into a piece of UGF uplift tube that had a piece of aquaclear foam cut to size and stuffed into the end. 
The co2 bubbles would get chopped into fine bubbles by the powerhead, and then shoot out into the uplift tube and get caught in the sponge until they were absorbed into the water. if my yeast mixture was feeling frisky, a 2" "head" of co2 would build up in the top of the uplift tube. Back then (we're talking in the 90's here) I'd never heard of a drop checker and had no way to measure the actual co2 in the water, but I never had bubbles of co2 make it to the surface, and my plants would pearl like crazy.

Measuring the co2 headspace in the uplift tube was also a easy way to monitor the health of the yeast mixture, and when the headspace began to shrink, it was time to start / switch in a new yest bottle. I believe this allowed me to keep a relatively even CO2 dosing going, without the normal yo-yoing that yeast is prone to.


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## MrCrabs (Nov 24, 2015)

Something related that I always wondered about when I researched CO2:

I never saw someone mention to use a simple ceramic diffuser and put it directly next to the intake of a canister filter.
Wouldn't that make the microbubbles get sucked directly into the filter and therefore increase contact time with water?

Is there anything that happens inside the filter that would remove the co2?

As a personal side note: I really like the look of the fine bubbles come out of the diffuser placed inside the tank - it's kinda elegant - so for some people it might not only about efficiency but about aesthetics too.


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## Argus (May 22, 2013)

MrCrabs said:


> Something related that I always wondered about when I researched CO2:
> 
> I never saw someone mention to use a simple ceramic diffuser and put it directly next to the intake of a canister filter.
> Wouldn't that make the microbubbles get sucked directly into the filter and therefore increase contact time with water?
> ...


I've read posts here where people claim they have an inline diffuser going into a filter. I have no actual experience with it, but would worry that there might be too much CO2 in the filter for the beneficial bacteria. It needs oxygen.

I don't like that the tiny bubbles give the aquarium water a cloudy appearance.


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## houseofcards (Mar 21, 2009)

Many do diffuse into the filter, but you can get noise from the air kicking around and probably not good for the impeller long-term. I would agree that efficiency-wise the reactor would be the way to go, but either method (inline reactor/diffuser or in tank) works and has been proven to work well for most setups. If your telling me that your tank is 6-ft+ then one ceramic diffuser might not be enough, you would need multiple with flow to move it, so most would choose in-line in that case. Co2 is cheap so I think preference and you setup would come first when deciding what method to use.


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