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Discussion Starter · #1 ·
Hello everyone,

I've been reading that the way oxygen(O2) enters my tank is through surface agitation (contrary to popular belief of air bubbles through air stones).
This is certainly something I want, to provide my fish with enough oxygen.

I have also read that a good source for my plants is to have CO2, but to prevent CO2 from escaping I need to reduce surface agitation.


Is it possible to achieve an equilibrium where I can have CO2 on the tank but allow O2 to enter? (I have an HOB filter)


As a side note, I have also read that it is important to stop CO2 at night (when there is no light source for photosynthesis), this because without lights, plants can't process CO2 and thus start consuming O2, competing with fish for O2 and eventually leading fish to suffocate due to lack of O2 and excessive CO2.
How accurate is this?


Thank you everyone.
 

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I've been reading that the way oxygen(O2) enters my tank is through surface agitation (contrary to popular belief of air bubbles through air stones).
Yes, this is correct. Air bubbles (through an airstone) simply causes surface agitation, increasing surface area and thus, gas exchange.

I have also read that a good source for my plants is to have CO2, but to prevent CO2 from escaping I need to reduce surface agitation.
This is also correct.

Is it possible to achieve an equilibrium where I can have CO2 on the tank but allow O2 to enter? (I have an HOB filter)
Yes, it is possible. You want enough surface movement so that the water is rippling (the surface of the water should not break). This will allow sufficient surface area for good gas exchange, while at the same time, not off gassing too much CO2.

As a side note, I have also read that it is important to stop CO2 at night (when there is no light source for photosynthesis), this because without lights, plants can't process CO2 and thus start consuming O2, competing with fish for O2 and eventually leading fish to suffocate due to lack of O2 and excessive CO2.
How accurate is this?
This is accurate. Most people turn off CO2 at night (using a solenoid on a timer) to prevent excessive CO2 build up. In addition, it helps conserve gas.

However, even if you do not have a solenoid, it is possible to leave CO2 running 24/7 (it just needs to be run at a lower rate).
 

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In an CO2 injected tank, agitation causes offgassing and lowers levels.

In a low tech setup, agitation increases CO2 levels.
 

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Off gassing occurs when the partial pressure of a gas in the water exceeds the partial pressure of the gas in the air in contact with the water. In gassing occurs when the opposite is true. The higher the differential in partial pressures the faster the exchange will take place. In an aquarium there are naturally occurring particulate at the surface, normally called scum, which will impede the transfer of gas. Surface agitation will reduce the scum and therefore facilitate the exchange process. The larger the surface area in contact with the water the faster the transfer will take place. The surface area in bubbles is quite small compared to the surface area at the top of the tank so better exchange takes place at the top surface than takes place due to bubbles, though bubbles do agitate the surface very well.
Given the above and knowing you want to maximize the dissolved CO2 during the light period and maximize the O2 during the dark you can manipulate the variables in order to achieve this. The variable with the greatest impact on the gas exchange process is the relative partial pressures involved. The normal approach is to achieve the best rate of dissolving of the CO2 in the water and then trying to slow the out gassing by not agitating the surface too much, yet at the same time no one likes to see the scum on the water's surface.
The method I use is to employ an in-line diffuser (I believe these give the smallest micro bubbles) on the intake to an external pump. The added agitation of the micro bubbles with the pump's impeller and the strong currents created by the pump provide a fairly high rate of dissolving of the CO2 in the water. My aquarium has a top on it which though not completely sealed does provide enough sealing to force the CO2 out gassing from the water to build up under the top thus raising its partial pressure in the air in contact with the water. The lower differential in partial pressures drastically reduces the out gassing process and could in theory reverse it and create an in gassing to take place. This method is so efficient that you need to turn down the CO2 flow rate and monitor your fishes' comfort closely. It is easy to get super high levels of CO2 this way. It also negates any concern regarding surface agitation, you can agitate the surface all you want and it will have little or no impact on the CO2 levels.
During the dark period you want the opposite. To achieve this the CO2 flow turns off automatically and then the CO2 that is under the aquarium's top is flushed out. This is done with an air pump & air stone on a timer bringing fresh air under the top. I now have extra surface agitation from the bubble column and the differential in the CO2 partial pressures is made to be relatively high forcing an out gassing of CO2 from the water to the air. There is an added benefit that the differential of the O2 partial pressures is also high (but in reverse, high in the air and low in the water) forcing an in gassing of O2 from the air into the water.
The process is offset to the change in the lighting period by an hour so the CO2 and O2 levels are high when they are needed to be.
 

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Discussion Starter · #6 ·
Thank you so much everybody, this is so much valuable knowledge.

Very helpful as in going through the fishless cycle and getting a nice planted tank at the same time.


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Just a quick correction. Increased gas exchange due to surface agitation is due to mostly the ripples. The ripples dramatically increase the surface area of the water in contact with the air. Easily more than 10x. Since the gas exchange is very fast the increased surface area has a huge effect.

Very small ripples that are close together give the greatest increase.
 

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I run my lily pipe about 1/3 above the water which does break the surface to increase gas exchange and completely free of scum accumulation, 24/7 co2. Yes, I have to use more co2 but I feel that the benefits of increased oxygen outweigh the wasted gas. It's also much harder to overdose co2 with lots of surface agitation.
 

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I use C02 with PH Meters from Milwaukee on my 75 and 90 gallon tanks. I keep the PH at 6.6 and it turns on when it needs to. In my 75 gallon I have air bubbles running 24x7. It doesn't seem to kick on my C02 all that more often than when I didn't run the bubbles. I did notice at a point that my fish were breathing much more heavily but weren't yet on the top gasping for air, so I turned the bubbles back on. The fish seem happier and I don't seem to be using that much more c02 than I normally would.


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Discussion Starter · #10 ·
Thank you for sharing, I will certainly keep it in mind as I learn and adapt to how my little tank behaves.

This is very helpful. You all are the greatest, thank you for taking the time to respond.
 
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