Will a surface skimmer off gas a ton of co2?

[philipraposo1982]

I want to get an eheim skim350. Will this have a negative effect on co2 levels?

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[JamieB]

No as any co2 that is on the surface wouldn't be absorbed into the water anyway

 

[philipraposo1982]

Really? Why wouldn't it be? I normally have lots of co2 bubbles at the surface.

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[dw1305]

Hi all,

Really? Why wouldn't it be? I normally have lots of co2 bubbles at the surface.

CO2 bit
It is because the bubbles are already at the gas exchange surface between water and atmosphere.

Carbon dioxide is very soluble in water, but it only makes up 400ppm of the atmosphere, this means that at <"normal temperature and pressure"> (1013mb and 20oC), only about 0.5 ppm CO2 is present in water (at equilibrium with the atmosphere).

We can add CO2 (because it is soluble), but at the waters surface it will escape back into the atmosphere along the concentration gradient between the level in the water and the air, and it will keep on escaping until the water reaches the equilibrium value again.

When your CO2 rich gas bubbles are at the waters surface they will quickly lose their CO2 to the atmosphere. Carbon dioxide won't diffuse back into the water, because the water already contains more CO2 than the equilibrium value.

Drop Checker and pH bit
Because a very small proportion of CO2 goes into solution as H2CO3 (an acid or "H+ ion donor"), and H2CO3 ~ HCO3- are in equilibrium (they are an acid & base pair in a buffered system), we can use pH drop to estimate our level of dissolved CO2, either in a drop checker (with a narrow range pH indicator and solution of known dKH), or in the tank water with a pH probe.

A drop checker works because there is an air gap between the water and the 4dKH solution, CO2 can diffuse into the drop checker and after a period it will reach equilibrium with the tank water, and the pH indicator will change colour to reflect the level of extra H+ ions (the extra H+ is from the dissolution of H2CO3 in to H+ and HCO3- ions). When we stop adding CO2 to the tank CO2 will be lost from the drop checker via diffusion back into the tank water which is now in equilibrium with the atmosphere, and the pH indicator will change colour to indicate the rise in pH caused by the loss of the "extra" H+ ions.

The pH probe will directly measure the change in the ratio of H+:OH- ions, and <"experiments have shown"> that a pH drop of 1 unit indicates about 30ppm CO2.

cheers Darrel

 

[Chris Jackson]

I want to get an eheim skim350. Will this have a negative effect on co2 levels?

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I have found that a skimmer helps stabilse CO2 levels if you do not have a lot of surface movement already, it certainly won't make CO2 addition a problem. If you have surface film then the CO2 tends to build up whereas the skimmer removes this film allowing more CO2 to escape and so the CO2 level stabises at a certain point rather than climbing higher and higher but more surface agitation will do a similar job.

Skimmers are a bit of a pain to keep clean however as every little bit of trimming debris etc. gets pulled in and there is always more than you expect! A sparkly clean water surface is of course very nice in an open topped tank.

 

[philipraposo1982]

Thanks for the feedback everyone. I am going to try one

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[andyone]

Really? Why wouldn't it be? I normally have lots of co2 bubbles at the surface.

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bubbles trapped in the scum layer hence why you want a skimmer

 

[philipraposo1982]

I got the eheim skim350 and it works amazingly.

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