# Co2 comparisons?



## JenCliBee (26 Sep 2011)

Hi all, have a quick question regarding lowtech setups and tap water in regards to Co2 amounts.


Right then lol..... i know how low tech setups work... the low maintenance, low flow, plants do all the work.. self contained ecosystem, none or very few water changes  etc etc..... 

I'm planning a low tech tank of sorts, ie... very few ferts (still using them due to low stock levels), no co2 etc.... now i dont do the whole no water changes im afraid becasue i feel that fish need fresher water than what is recommended for a low tech setup.... however ive seen that when people suggest still doing water changes. (DW for instance), they suggest leaving the water to gas off most of the co2 within that water to get it more to the tank levels to avoid fluctuations... i get this.... BUT.... does this apply only on a lowtech tank that has very little flow and no surface agitation?.

Reason i ask... i would be having surface movement which i presume will bring atmospheric co2 in the tank, the agitation would be constant so i presume the levels within the tank would also be quite stable even with uptake from plants, would tap water straight off contain more co2 within it, than my tank with constant surface movement?

Or would tap water still contain much more co2 than what my tank would take atmospherically?


I hope this made sense lol, i really cant think of how better to explain my question   

Thanks in advance 


Jen


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## dw1305 (26 Sep 2011)

Hi all,


> now i dont do the whole no water changes im afraid becasue i feel that fish need fresher water than what is recommended for a low tech setup.... however ive seen that when people suggest still doing water changes. (DW for instance), they suggest leaving the water to gas off most of the co2 within that water to get it more to the tank levels to avoid fluctuation


Jen, like you I'm not convinced about this whole "fluctuating CO2 and not changing water" thing at all, and personally I'm pretty convinced that all tanks benefit from regular water changes. In my tanks (I use rain-water but it is buffered to some degree) I'll find that the pH will be some-where down towards pH6 before lights on, and well above pH7 during the photo-period. This can only be caused by fluctuating CO2 levels, and this diurnal cycle of "_CO2 build up and depletion_" must happen in nearly all water bodies with plants growing in them, even if pH changes are masked in heavily buffered water. 

The reason that the levels of CO2 are higher in tap water is that the water from the tap will both be cooler than your tank water, and under pressure in the water main, both factors meaning that it can hold more gas (if that doesn't make sense look up "Henry's Law" on Wikipedia). 


> Reason i ask... i would be having surface movement which i presume will bring atmospheric co2 in the tank


Yes, I think this is true. If you aren't adding CO2 the larger your gas exchange surface is the quicker that the levels of all gases will equilibrate with the atmospheric level. However I think with reasonably high PAR the plants will manage to deplete the CO2 in almost all systems, even in thin streams of rapidly flowing water. 

I've always used trickle filters, and/or massive over filtration, to ensure that the water remains as fully oxygenated as possible, but this will also apply to CO2, the only real difference is that CO2 is more soluble in water than O2 (but obviously atmospheric CO2 is at much, much lower concentrations).

cheers Darrel


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## vauxhallmark (26 Sep 2011)

Just put an airstone in the new water for a day before you put it in the tank - should have a similar effect to the surface agitation you have in your tank and reduce all dissolved gases to their natural concentration. 

I don't think you can get co2 to dissolve into tank water through surface agitation or airstones - it's more usually used as a method of reducing co2.

Mark


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## Alastair (26 Sep 2011)

I'm with darrel, in my previous tank, I didn't run co2 or ferts etc, had lots if plants and did my water change every Saturday 25percent and never saw any problems with algae etc. I was always under the impression that for fish health it was best to do a regular water change. Fish loved it too


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## dw1305 (28 Sep 2011)

Hi all,


> I don't think you can get co2 to dissolve into tank water through surface agitation or airstones - it's more usually used as a method of reducing co2.


Mark, it depends upon the levels of CO2 and in the tank water and in the atmosphere. If you don't add CO2 to the tank, and the plants remove it during photosynthesis, CO2 will then diffuse into the water (through the gas exchange surface) until equilibrium with the atmosphere is reached.  

Usually we are worried about CO2 escaping because the ~ 20ppm CO2 level suggested for improved plant growth is about 40 times the natural atmospheric content. If we have efficient gas exchange, either by water movement or filter design, CO2 will dissolve out until it equilibrates with atmospheric CO2 levels. Unless we are physically adding CO2 to the water, water movement, and trickle filters, will add CO2 to the water when plants are actively photosynthesising (utilising CO2 and producing oxygen), and out-gas CO2 when levels in the water exceed those in the atmosphere. 

The atmosphere has about 0.04%  (400ppm)  of CO2, but it contains ~ 20% O2 (200,000 ppm), so even though CO2 is more soluble than O2, it occurs several orders of magnitude less frequently in the atmosphere. The solubility of both CO2 and O2 will depend upon the temperature, the warmer it is the less gas the water can hold. 

cheers Darrel


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## JenCliBee (28 Sep 2011)

> Jen, like you I'm not convinced about this whole "fluctuating CO2 and not changing water" thing at all, and personally I'm pretty convinced that all tanks benefit from regular water changes. In my tanks (I use rain-water but it is buffered to some degree) I'll find that the pH will be some-where down towards pH6 before lights on, and well above pH7 during the photo-period. This can only be caused by fluctuating CO2 levels, and this diurnal cycle of "CO2 build up and depletion" must happen in nearly all water bodies with plants growing in them, even if pH changes are masked in heavily buffered water.
> 
> The reason that the levels of CO2 are higher in tap water is that the water from the tap will both be cooler than your tank water, and under pressure in the water main, both factors meaning that it can hold more gas (if that doesn't make sense look up "Henry's Law" on Wikipedia).




All makes perfect sense mate and thankyou for the info 




> Yes, I think this is true. If you aren't adding CO2 the larger your gas exchange surface is the quicker that the levels of all gases will equilibrate with the atmospheric level. However I think with reasonably high PAR the plants will manage to deplete the CO2 in almost all systems, even in thin streams of rapidly flowing water.
> 
> I've always used trickle filters, and/or massive over filtration, to ensure that the water remains as fully oxygenated as possible, but this will also apply to CO2, the only real difference is that CO2 is more soluble in water than O2 (but obviously atmospheric CO2 is at much, much lower concentrations).
> 
> cheers Darrel



So basically the water added would still be better off gassed of most of it's co2 to avoid big fluctuations within the tank?. So even having huge disturbance of the water surface wouldn't bring in the amount of co2 the plants would consume. That's pretty much what my original post tried to get at. Thanks for confirming   




> Just put an airstone in the new water for a day before you put it in the tank - should have a similar effect to the surface agitation you have in your tank and reduce all dissolved gases to their natural concentration.
> 
> I don't think you can get co2 to dissolve into tank water through surface agitation or airstones - it's more usually used as a method of reducing co2.
> 
> Mark



Cheers for the reply Mark (by the way, great name ), i think from Darrel's reply, still gassing off as you suggested would be a much more tank and plant friendly option.




> I'm with darrel, in my previous tank, I didn't run co2 or ferts etc, had lots if plants and did my water change every Saturday 25percent and never saw any problems with algae etc. I was always under the impression that for fish health it was best to do a regular water change. Fish loved it too



Thanks for the reply , i have to agree, ive kept fish now for a very long time and water changes perk fish up no end, im not convinced on low tech tanks that no water changes can be good for fish (plants maybe), i would imagine in every 'body' of natural water, there would be some process of water change.. whether it with heavy down pours or natural process through steams etc etc. I always do a water change as fish health for me is far more important than plants health... people may disagree with this but each to there own as with most aspects of the whole hobby.




> Mark, it depends upon the levels of CO2 and in the tank water and in the atmosphere. If you don't add CO2 to the tank, and the plants remove it during photosynthesis, CO2 will then diffuse into the water (through the gas exchange surface) until equilibrium with the atmosphere is reached.
> 
> Usually we are worried about CO2 escaping because the ~ 20ppm CO2 level suggested for improved plant growth is about 40 times the natural atmospheric content. If we have efficient gas exchange, either by water movement or filter design, CO2 will dissolve out until it equilibrates with atmospheric CO2 levels. Unless we are physically adding CO2 to the water, water movement, and trickle filters, will add CO2 to the water when plants are actively photosynthesising (utilising CO2 and producing oxygen), and out-gas CO2 when levels in the water exceed those in the atmosphere.
> 
> ...



All makes much more sense once explained like that.


Thanks again all for the input, all very useful and has answered my question perfectly .


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## Brenmuk (28 Sep 2011)

dw1305 said:
			
		

> Hi all,
> 
> 
> > I don't think you can get co2 to dissolve into tank water through surface agitation or airstones - it's more usually used as a method of reducing co2.
> ...



For me the key phrase here is 
'it depends upon the levels of CO2 in the tank water and in the atmosphere'.

The term low tech covers a wide range of tanks and is not a exact definition. For example a low tech tank using a soil  layer a la Walstead style tanks, with organic matter in the soil, will have higher levels of CO2 than a tank that does not. Also the bio load will be producing CO2 all the time, ie filter bacteria, substrate bacteria, plants and fish/inverts etc so before lights on and photosynthesis kicks in the levels of CO2 in the tank would in theory (I've not measured this) be higher than that achieved by equilibration with air. So maybe some additional aeration might be useful some time after lights on only when CO2 becomes depleted?

In a deep low tech tank with just sand or gravel substrate then maybe good water turnover combined with good aeration all day would help get the CO2 to the hard to reach places. I've seen loads of tanks like this where all the plant growth occurs at the surface and the substrate is a plant free zone.

From my own experience I have not noticed any deterioration in fish/invert health by not doing regular water changes as long as the plants are growing well. I have noticed a decline in plant growth and the appearance of algae in my tanks if the flow from the filter slows down so every few months or so I clean the filter and do a 20-30% water change.  
I think its a personal preference about the quantity and frequency  of W/C in low tech tanks they are after all supposed to be more forgiving in their maintenance routine than high tech tanks where W/C are critical. 

It was also difficult mindset for me to change after reading for so many years about the importance of regular large water changes - this i think still applies to fish only tanks or tanks that cannot support good plant growth.


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## dw1305 (28 Sep 2011)

Hi all,


> Also the bio load will be producing CO2 all the time, ie filter bacteria, substrate bacteria, plants and fish/inverts etc so before lights on and photosynthesis kicks in the levels of CO2 in the tank would in theory (I've not measured this) be higher than that achieved by equilibration with air.


 Yes, it definitely is. You could even estimate the amount of CO2 present by the depression of the pH. In water with very little carbonate buffering and active plant growth you can get absolutely huge diurnal swings in pH as the biomass uses O2 and produces CO2 during the night, and then the water becomes saturated with oxygen and the CO2 depleted during photosynthesis. Diana Walstad has a graph to illustrate this at the start of Chapter VI "Carbon", fig. 6.1 pg 91of "Ecology of the Planted Aquarium" (after Boyd 1995 "Bottom soils, sediment and pond aquaculture" Chapman Hall). 


> So maybe some additional aeration might be useful some time after lights on only when CO2 becomes depleted?


 It certainly won't do any harm and it might produce a large enough increase in dissolved CO2 to produce a noticeable growth response.


> In a deep low tech tank with just sand or gravel substrate then maybe good water turnover combined with good aeration all day would help get the CO2 to the hard to reach places. I've seen loads of tanks like this where all the plant growth occurs at the surface and the substrate is a plant free zone.


 I'm not convinced this is a CO2 response. I think it is more likely to be a combination of lower light intensity and low nutrients, both of these factors will tend to make the plant put its resources into growing new leaves towards the waters surface and with-drawing mobile nutrients from the older leaves, before discarding them. 


> From my own experience I have not noticed any deterioration in fish/invert health by not doing regular water changes as long as the plants are growing well. I have noticed a decline in plant growth and the appearance of algae in my tanks if the flow from the filter slows down so every few months or so I clean the filter and do a 20-30% water change. I think its a personal preference about the quantity and frequency of W/C in low tech tanks they are after all supposed to be more forgiving in their maintenance routine than high tech tanks where W/C are critical.


[/quote]
This is certainly true to some degree, theoretically you should be able to fully filter water in a recycling planted system.  We did have a go at a closed system with the landfill leachate, and while it works fairly well on a full size system (the landfill has a small "sewage works" and vertical reed bed filter), it didn't work so well scaled down. I'm not sure why this was, we never managed to find the factor(s) that led to declining water quality over time.

cheers Darrel


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