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

Discussion in 'CO2 Enrichment' started by Tom Barr, Jul 6, 2012.

  1. Tom Barr

    Tom Barr Founder
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    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. dutchy

    dutchy Plant Guru Team
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    So the scum layer and low O2 is related? What is "low" O2 in this case?
     
  3. dantra

    dantra Lifetime Charter Member
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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 :mad-new:). 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 O[SUB]2 [/SUB]the tank needs.

    I'll keep you guys posted of the results.
     
  4. Tom Barr

    Tom Barr Founder
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    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.
     
  5. dantra

    dantra Lifetime Charter Member
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    I found this to be interesting. BOD
     
  6. Tom Barr

    Tom Barr Founder
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    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.
     
  7. mike

    mike Guru Class Expert

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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. Tom Barr

    Tom Barr Founder
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    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 $.
     
  9. Yo-han

    Yo-han Guru Class Expert

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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...
     
    #9 Yo-han, Jul 12, 2012
    Last edited by a moderator: Jul 12, 2012
  10. Tom Barr

    Tom Barr Founder
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    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.
     
  11. samh

    samh Guru Class Expert

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    hi tom do you have video of the flow and surface agitation in your 120gal?

    sam
     
  12. Tom Barr

    Tom Barr Founder
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    The 180 does.
    Same thing.
     
  13. Matt F.

    Matt F. Lifetime Charter Member
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    informative thread. Thanks
     
  14. pejerrey

    pejerrey Prolific Poster

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    Very informative! Thanks!

    I was on my way to post a video of my main tank surface agitation in a thread I have in other forum. I was going to post it because of the concern some members have of suffocation of the livestock when using 24/7 co2 (I don't believe is better than timed, I have other reasons to experiment with 24/7 :)), and I wanted to have a more clear idea for them to see how i do it.

    So, now that I'm reading this. I would assume that the reason why I can get away with "less" co2 now is because I really didn't have anything near this kind of surface agitation in my previous set up. Am I understanding this right?
    I was considering that maybe non stop injecting co2 was allowing me to have lot less bps.

    I wanted to ask here first if this looks like enough surface agitation to show as an example for this set up?

    I don't like to just post info without some kind of research back up. :)

    Thanks!

    Edit: I understood the first link provided in the first post but the second one I don't have the knowledge to decipher. How does this apply to the UGFs? (in regular joe English please;)) I'm just assuming that the law cited makes the UGF claim of "improvement of root nutrition/co2/o2" kinda false as when added to transpiration flux will suffice to bring nutritious water down while o2 is provided by the roots. Right?
    I wonder then if the claim of too much o2 for the roots because of using a UGF is kinda true?
    I'm glad I stumbled upon this thread! :D
     
    #14 pejerrey, Jul 23, 2012
    Last edited by a moderator: Jul 23, 2012
  15. Tom Barr

    Tom Barr Founder
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    Might be a tad over doing it Carlos.
    The clean water surface, good, the rippling, good, the outflow where the return is, might want to adjust the flow more to the horizontal and less up into the surface.

    UFG= undergravel filter. RFUG= reverse flow UG etc. Marine folks have a huge array of "acro speak" as well.
    Well, we all do in the narrow areas of various hobby/careers etc.

    Roots like a mix of reduction and oxidation, they modify the sediment to form this environment, adding UGF's, RFUG, Heat cable,s Powersand etc.........none of those do that nor foster plant root health anymore than the plants do themselves.

    In otherwords, plant define both the sediment and the water column. Not engineering mechanical type stuff like cables/RFUG's, not large pieces of pumice, not various additives. Not PO4, not NO3, not Fe etc...........for the water column.
    These can be over a extremely wide range and plants will grow well and be algae free.

    You cannot argue that there is some fine ppm range or flow characteristic through the sediment etc and have a successful planted tank. It's horse crap.
    Plants do that job, not the guys in marketing etc.

    It's a philosophical divide. One looks at the plants as the defining ecosystem, others think nutrients and mechanical devices.
    Some assume the mechanical and some obscure narrow dosing of ferts is "natural". I'm afraid many of these folks have not really looked at natural systems much.

    You might have had a situation, that the tank was not limited so much by CO2, rather, by better O2.
    Seems to explain what I've seen in several tanks, where the CO2 is verified pretty well.
    Co2 is still important...........but so is O2.
     
  16. pejerrey

    pejerrey Prolific Poster

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    Its funny I didnt know any of this myths about UGFs or cables! lol!
    I did it as suggested by Liam for my shrimp, and I was happy that it didn't really affect negatively my plants as far as I can account for.
    I did that with the spraybar to ensure oxygenation based on the benibachi shrimp factory, but I did it from under the surface to keep the Co2. I will adjust as suggested tho.
    Now, after reading this thread the difference between both set ups make more sense, lack of O2 is not mentioned as a limiting factor for CO2 all over the place. I'm amazed of how may things are not taken into account in the hobby. In Planted Tanks there is more Myth than in the Old Greece. :D
    Thanks!

    Edit: Oh, and where do I put the Horse manure? under the UGF or in top of the UGF? LOL! :D (joke)
     
    #16 pejerrey, Jul 24, 2012
    Last edited by a moderator: Jul 24, 2012
  17. David

    David Prolific Poster

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    Interesting thread.

    I have seen mention of adding a small air pump with an airstone/diffuser set to run on a timer at night, while a timed CO2 system is off.

    Will the dissolved O2 levels from overnight aeration help with gas exchange throughout the day, or at least help with maintaining dissolved O2 in the presence of a layer of surface scrum from not having a wet/dry filter? On the same note, could an airstone be added to run 24/7, in conjunction with a CO2 diffuser during the photoperiod?
     
  18. Tom Barr

    Tom Barr Founder
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    I'll use some of the experiences I have from chemical dosing and electric pulses to show why and how this is important graphically.
     
  19. jcgd

    jcgd Prolific Poster

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    I may have misunderstood something in the explanation, but I only see why two given tanks may have different injection rates.

    But why would two tanks required different levels of co2. Regardless of the injection rate, why would your 180 need 75ppm, while your 120 needs 50ppm? Is the only variable the plant species and co2 distribution and their individual abilities to compete for co2, or are there other reasons besides that?

    I can understand that one tank with surface scum may maintain a level of 30ppm with less co2 being injected than a tank with a surface skimmer and no scum maintaining 30ppm. I don't understand why the needs from two tanks with the same species would require more or less co2? Or would it if everything else (flow, co2 distribution) were identical?
     
    #19 jcgd, Jul 24, 2012
    Last edited by a moderator: Jul 24, 2012
  20. Matt F.

    Matt F. Lifetime Charter Member
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    Not to over simplify this answer on purpose, but no two tanks are every going to be exactly the same...even with the same filter, same plants, lights, etc. There are too many variables at play. A planted tank is dynamic. So are CO2 levels, temperatures, a plant's metabolism, the amount of bacteria breaking down wastes at any given time etc.
     
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