# Thread: CO2: why some tanks might need more CO2 than others and why some folks gas their fish

1. ## CO2: why some tanks might need more CO2 than others and why some folks gas their fish

If you look at the pics and video of my tanks, they are rippling good so there is PLENTY of O2. There is zero surface scum. Surface scum will retain CO2 and this is good if you add too little, as you add more or if the scum comes and goes as is often the case for many...........then this can gas the fish by not allowing it to degas as it gets higher ppm. As the concentration of CO2 increases in the water relative to air above, so does the flux. Same with the thickness and type of boundary layer. If this boundary layer is reduced in thickness and/or the scum is removed allowing from much better/more stable CO2/O2 exchange, then the ENTIRE SYSTEM IS FAR MORE STABLE OVERALL.

Some examples and these can be used to estimate things like sediment flow through and heating cables, UG filters etc.

http://en.wikipedia.org/wiki/Fick%27s_laws_of_diffusion

http://omlc.ogi.edu/classroom/ece532/class5/ficks1.html

J= -D * Concentration difference/distance of boundary layer

If the type of boundary layer is going from clear water with no surface scum layer to one with a scum layer, either whoile or partial, this GREATLY will effect the flux.
The distance change is still relative very small vs the two types of boundary layers.

So the equation reduces mostly to: J= -D * type of boundary layer/a set distance.

The equation predicts:
As the concentration of CO2 increases(say 10ppm to say 50 ppm), the tank with less/no scum layer will diffuse out FASTER than the tank with the layer.
As the CO2 is reduced, this difference is reduced. This is because the concentration difference between the air and the water is reduced.
This may explain why there are large differences in CO2 in tanks with higher flow and wet/dry filters vs tanks with canister filters which tend to have scum layers, visible or not ....and lower O2 levels.
Scum layers come and go and may often appear after night and then fade away after a few hours in the morning.
Some tanks have long term persistent scum layers.

These tend to have poor O2 and a few have larger O2 swings daily and higher CO2 ppm at night.

2. So the scum layer and low O2 is related? What is "low" O2 in this case?

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Tom this is very interesting considering I am at the verge of revamping my tank. I purchased a hydor koralia which you mention to me some time ago to get. (I should have listened a long time ago ). I'm going to keep the loc-lines low toward mid-section of the tank and use the koralia to provide surface movement for gas exchange. I believe this will provide the O2 the tank needs.

I'll keep you guys posted of the results.

4. Originally Posted by dutchy
So the scum layer and low O2 is related? What is "low" O2 in this case?
6.0ppm or less IME.
Still okay, but when I run 7-9ppm consistently.........

This is at 29C

O2 is used up and the flux is decreased, so it never quite gets up to say 7ppm.

Is this a minor issue?
Perhaps, till you add more and more CO2, then the 15% more O2 is not. The counter to this is that the O2 produced by plants above ambient is retained longer, but this is only for a few hours at night, not 24/7.

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I found this to be interesting. BOD

6. Yes, plant tanks have much higher BOD/COD than typical fish only tanks.
there's just a lot more organic matter, many have soil etc...........dead leaves alone are huge..........

So this needs countered IMO with good current/Wet/drys, removal of scum layer etc.
Then while the BOD/COD is the same, the rates of cycling are much faster and better, less decline in O2, less over dosing of CO2 since it can escape easier in the tank.

You want to reduce the CO2 from escaping too much, and then escaping too little.
That's the balance.

What makes it the most consistent method wise? No scum layer. More O2.
Wet/drys do both.

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I think right now I'm fighting with the proper balance of CO2 and O2. I still have a slight problem with BBA and I can't put anymore CO2 in without my fish staying at the top. All my plants sway so I figure I have good circulation and I also have noticeable surface movement/ripple.

You say 6ppm or less of O2 is considered low, the question is how do you measure O2?

Thank you,

Mike

8. They make DO meters.

I happen to have a Hach HQ 40 with a LDO probe that can data log every few min.

But they have others, folks can nab them off ebay etc for sometimes a decent \$.

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Been expecting high co2/low O2 as a cause of my scum layer for a while. This mainly because it is very present in the morning and almost non existing in the evening. My plants assimilate a lot during the day and I run co2 24/7 on a pH controller. My other tank which uses co2 on a timer, never has a scum layer. I still suspected high co2, until I noticed that a tank on my work had less scum when the lid was left open (co2 24/7). I adjusted the pH on my big tank for a few days by hand by raising it after lights out and putting it back before lights on and noticed significant less scum. That's how I figured out it might be a lack of O2. Off course this is not doable for the long run, so I bought a new co2 regulator with a magnet valve for my big tank and placed in front of my ph controller and run this magnet valve on a timer. I think this will make a big difference but it is just installed and too soon to jump to conclusions.

Off course I was wondering about the theory behind it but nobody really seems to know what the scum layer is build off. I found statements about dead bacteria, life bacteria, proteins and lipids. It will probably be a mixture of those, but dead/life bacteria was the only one I could explain by my observations. Aerobe bacteria in the filter die (migrate to a place with more oxygen like our surface) because of a lack of oxygen in the filter (in my case, mainly at night). Is this true or does someone else done research on what this layer contains?

PS. Can try to add a bit of this layer on to an agar plate:P This at least confirms or not whether it are life bacteria...
Last edited by Yo-han; 07-12-2012 at 01:48 PM.

10. I think what precisely is in the scum layer is unknown, but plant's produce a fair amount of organic material and excrete a significant amount of carbohydrates. Root zones are highly influenced by plant's above and their addition of O2.
During the day, these root zones are enriched with O2 via the plant roots.

At night, not much is added by the plants.

Each tank likely has a different mix but there's likely a general mixture that comprises the scum layer.
We do know it can come and go and seems less when plant growth is most intense and the O2 ppm rises/goes up.
Bacteria respond very well to increases in O2 in the ranges we have(say 70-130%). If you have stable O2, say 100-130%, then it's not likely you have much scum anytime of day.
Only way to do that is with a wet/dry filter, surface skimming etc.

But..then you do not have a scum layer anyway.

So while it might be of interest, knowing what the layer is..may not be that critical.

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