# C02 and filters



## potatoes (Jun 25, 2010)

Hi,
I am trying in vain to find a filter for my ten gallon NPT shrimp tank that will minimize surface tension. All i find are air driven filters and HOB. I have been getting mixed messages from various sources regarding co2 and whether to maximize or minimize surface tension in the tank.

the two arguments are:
Surface tension is good because it keeps co2 at a consistent atmospheric level, and if there was none, the plants would use up the co2 too quickly and then not be able to get more from the atmosphere due to lack of surface tension.

the other is: Surface tension is bad because in a NPT, the decomposition of fish food, dead plant matter, and fish waste, all of which is carbon based, will break down and release co2. surface tension will release the precious CO2 and bring it down to atmospheric levels.

I guess there are two key questions.
1) Is a consistent atmospheric level of co2 sufficient in a low tech tank?

2) Is the decomposition releasing co2 at a rate faster then the plants can use it and keeping the co2 levels above atmospheric levels? If so, one would want to reduce surface tension and feed a lot (which Diana advocates). If not, one wants to maximize it.

Perhaps the situation is different in different tanks. 
Maybe in a low stock tank, such as the aforementioned shrimp tank, there will be minimal feeding and therefore minimal decomposition and less co2, so surface tension may be adventagious because the atmospheric levels may be higher then the levels created by the natural decomposition. 
Maybe in a more heavily stocked tank with liberal feeding, there is higher rate of decomposition and the rate of co2 release is higher then the constant atmospheric levels.

What do you guys think?


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## penfold (Dec 7, 2010)

That's a good question. Check this thread to find some CO2 measurements from Diana's tanks. She reports levels from 4-13 ppm in the morning and 0-5 ppm 12 hours later. An experiment here found a CO2 equilibrium level of 2-3 ppm. I guess this means that tanks may spend part of the day well above equilibrium, but could drop below equilibrium later in the day. I'd probably try to minimize surface turbulence, but you'd have to test your own tank to be sure of its particular tendencies.

For a 10g tank, I would just run an AquaClear filter. It takes up very little space in the tank and allows you to easily drop in a bag of carbon if you need to. You can minimize surface gas exchange by keeping the tank water level above the filter's output. There will be a slight surface ripple in front of the filter, but nothing too significant.


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## penfold (Dec 7, 2010)

After doing a bit more research, it seems that most CO2 in natural waters and aquariums is the result of biological processes like decomposition and fish respiration. Sterile water at atmospheric equilibrium only has about 0.5 ppm CO2. So it would make sense to feed sufficiently, and then minimize gas exchange (so long as enough oxygen is provided).


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## potatoes (Jun 25, 2010)

Thanks for the replies, that is extremely helpful. My research only found one source regarding the equilibrium concentration of co2, at it too said it was about .5 ppm. I dont know much about co2 requirements, but that doesnt sound like a lot. However, if it is at equilibrium with the air, then the air would have the same concentration, and terrestrial plants do not seem inhibited by a lack of co2. would a continuious level of .5 ppm be enough for aquatic plants at a low tech level? Could one reduce their lighting to an appropriate level and see slow but continuous growth? If not, why would it be enough for terrestrial plants? Perhaps there is some key chemistry thing i am missing. The source i found for the information regarding the co2 equilibrium level also said that that level was enough for good aquartic plant growth. i found the info here: http://www.hallman.org/plant/huebert.html . I do find it odd though the difference between the equilibrium levels found by this report and by diana.


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## penfold (Dec 7, 2010)

The equilibrium level of 0.5 ppm is for sterile water with no biological activity. In natural and aquarium waters there is a lot of biological activity which produces CO2 faster than it can diffuse into the atmosphere, so you typically end up with much higher levels as Diana found. There is typically significantly more CO2 in most natural waters than in the atmosphere for this very reason. The problem for submerged plants is that CO2 diffuses much more slowly in water than in air. The first paragraph of the link you provided describes it in more detail.


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## Jeffww (May 25, 2010)

potatoes said:


> Thanks for the replies, that is extremely helpful. My research only found one source regarding the equilibrium concentration of co2, at it too said it was about .5 ppm. I dont know much about co2 requirements, but that doesnt sound like a lot. However, if it is at equilibrium with the air, then the air would have the same concentration, and terrestrial plants do not seem inhibited by a lack of co2. would a continuious level of .5 ppm be enough for aquatic plants at a low tech level? Could one reduce their lighting to an appropriate level and see slow but continuous growth? If not, why would it be enough for terrestrial plants? Perhaps there is some key chemistry thing i am missing. The source i found for the information regarding the co2 equilibrium level also said that that level was enough for good aquartic plant growth. i found the info here: http://www.hallman.org/plant/huebert.html . I do find it odd though the difference between the equilibrium levels found by this report and by diana.


Not exactly. Although gasses are infinitely soluble in water they will equilibrate not only on concentration but on partial pressure and temperature. As kinetic energy increases gas molecules will escape the water which explains why you see bubbles when you tap a cold glass of water from the sink and let it sit. Gas also exerts pressure on everything- that's atmospheric pressure; the higher the atmospheric the more gas gets "pushed" into the water. Partial pressure is the pressure of an element ALONE that it inherently exerts based on its molar concentration and also temperature and what it is made of. The gas's partial pressure must be at a certain tipping point to dissolve (source: http://en.wikipedia.org/wiki/Partial_pressure#Henry.27s_Law_and_the_solubility_of_gases ). In order for gas to enter water there must be: 
1.) Sufficient partial pressure of the gas. 
2.) Sufficient atmospheric pressure.
3.) Reasonable Temperature 
4.) A Gradient (always must be, atmospheric CO2 is in the [edit: It's hundreds, I was thinking of oxygen ppm] of ppm)

For CO2 there is the added effect of equilibrating the H2CO3 <-> H2O + CO2 reaction.

In my opinion it's best to keep the water stagnant. Remember: When you in gas CO2 through a disturbing method you out gas much more. CO2 likes being in the air; it's more roomy.


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## potatoes (Jun 25, 2010)

Very interesting, thanks for that, it will take me a while to figure it out lol. So do you think that stagnant water is still better in a lightly stocked shrimp tank?

I got my hands on a co2 test from my bio teacher today and immediately took some measurements of some tanks. In the NPT i set up two weeks ago with the aquarium club i started, I measured 9ppm of CO2. the lights had already been on for several hours, and the tank is still new, so there is not significant decomposition and no fish to respire. in a tank that has a powerful HOB with tons of surface agitation, but no fish or plants, I measured the co2 at 13ppm. I am going to measure the co2 tomorrow morning and see if it is higher. I also hope that once the tank is established and has lots of decomposition, there will be more co2. if the co2 remains consistently lower then the tank with lots of agitation, i may maximize the agitation until the tank is more established and i get more decomposition.


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## penfold (Dec 7, 2010)

We've established that water at atmospheric equilibrium has a CO2 concentration of roughly 0.5 ppm, and that any additional CO2 would be from biological processes. So aerating water will always drive the CO2 concentration towards 0.5 ppm. There would be no benefit to aerating (in terms of CO2), unless your CO2 concentration is below 0.5 ppm.


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## potatoes (Jun 25, 2010)

Oh ok that makes sense.Thanks for answering my questions in such detail, it has been extremely helpful. I just tested three of my tanks to compare to the tests i took this afternoon at my school, because the tests i took this afternoon conflicted with this logic. I found that my month-old 29gal NPT with no fish had a co2 concentration of 9ppm, my lightly stocked 10gal and my heavily stocked 10 gal, which both had a lot of surface agitation, both had concentrations of 2 ppm. I will retest my teachers tank to double check it because 13ppm seems unusually high now.

Darn. I was hoping that my understanding on lightly stocked NPTs would have been somewhat accurate, because finding a filter for a 10gal that maximizes surface agitation is easy (HOBs, Sponge filters,...), but one that minimizes it is difficult because most powerheads are too powerful. Any suggestions? I cant find anything that would do this in any pet shops


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## penfold (Dec 7, 2010)

There are a number of variables that affect CO2 concentration. Fish load and surface agitation are two you mentioned. The substrate can add CO2 if it has considerable organics. And maybe most importantly, plant growth can have a large impact on CO2. Tanks with very strong plant growth can have their CO2 levels drop near zero during the day, while tanks with poor to moderate plant growth can have very high levels.

Also, gas exchange is not all bad. This same process that drives off excess CO2, also brings in new O2 for fish and bacteria. So you do not want to completely eliminate gas exchange, but minimizing it is probably a good idea as long as your fish are still doing alright. Personally, I would just use a HOB with the tank water level kept above the output to eliminate splashing. But if you wanted to go further, you could use an internal filter kept low in the tank, or no filter at all. Plants are entirely capable of removing all nitrogen compounds, and there's lots of surface area within the tank for nitrifying bacteria. A filter just gives you a way to polish the water by adding some filter floss and/or carbon. Though a small powerhead kept low in the tank may still be desirable in order to keep the water circulated.


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## potatoes (Jun 25, 2010)

Great, thanks. My only problem with HOBs is that there is a much narrower range of where the water level should be. We have our heaters on in the winter, which dries out the air and causes the tanks to loose a lot of water, so in order to keep the water at the optimum level, we would need to top it off quite often. This is not terrible, but if it is avoidable by using another type of filter, i would like too.
You have a very good point about not all gas exchange being bad; perhaps i am making to big of a deal out of it. I will try to see if i can find a suitable internal filter or powerhead online, otherwise i will get a HOB


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## Tuiflies (Jan 21, 2010)

If you add a bit of water frequently, I wouldn't worry about treating it. There won't be enough chlorine to harm anything. (Unless you add something to increase hardness or some other variable.) Adding a cup of tap water everynight at feeding time may be all you need to offset evaporation.


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## potatoes (Jun 25, 2010)

Thats actually a great idea. I would do that, but I am preparing ot leave for college and so i am trying to make my tanks as low maintainence as possible, and if i can reduce the maintainace with a internal filter thats what i would like to do. I used to do 30% weekly water changes, but i knew that would not fly when i dumped all my tanks on my parents, hence the switch to El Natural walsted low tech tanks.


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