Can't Believe I'm Asking This

[stu40]

This smacks of a being completely clueless, but what's the deal with surface agitation and gas exchange?
I understand why in a high tec setup when you're adding gas you don't want to lose any co2 by over agitating the water surface.
But what about in a 'low tec' setup with no carbon addition and longer periods without a water change?
Will you be adding or losing co2 by breaking the surface? Never really got my head around this one.
Anyone?

 

[StandbySetting]

Most probably losing CO2, as the fish and plants respire the concentration of CO2 gas in the water column increases. You have to remember that CO2 in this instance moves along a concentration gradient, going from high low to restore the equilibrium, so if the concentration of CO2 in your tank is higher than in the air surrounding it then it will diffuse into the air, the rate of diffusion will be faster if you're agitating the surface.

 

[stu40]

so if the concentration of CO2 in your tank is higher than in the air surrounding it then it will diffuse into the air, the rate of diffusion will be faster if you're agitating the surface.

Thanks Jack
i understand this, so i obviously the concentration of co2 in the water is directly related to fish stocking levels & how heavily planted the tank is.
So if you had minimal of both, in theory, agitation would be adding co2 (not that you'd need it in this example)?

 

[SO19Firearms]

Not sure it's as simple as all that. It's definitely a numbers game though.
I always understood not moving the water much in low tech to try and keep stable CO2 levels or levels that change very slowly.
In hightech you're actively maintaining 30ppm regardless of what the surface is doing or the surface area of the tank as your balancing your bubble rate with the drop checker

CO2 in air is about 4-500ppm I think, so I'm not sure its an equilibrium issue. Unless I'm wrong, I often am.

 

[StandbySetting]

so if the concentration of CO2 in your tank is higher than in the air surrounding it then it will diffuse into the air, the rate of diffusion will be faster if you're agitating the surface.

Thanks Jack
i understand this, so i obviously the concentration of co2 in the water is directly related to fish stocking levels & how heavily planted the tank is.
So if you had minimal of both, in theory, agitation would be adding co2 (not that you'd need it in this example)?

I really can't tell you, as it depends on numerous factors such as a the rate of photosynthesis, but in a low tech tank surface agitation isn't an issue anyway as the demands for CO2 from plants shouldn't exceed the concentration of CO2 in the water column.

CO2 in air is about 390PPM, so my previous post needs amending as the concentration gradient doesn't apply itself in the same way as it would if the two bodies were the same ie; air and air. Water isn't able to retain CO2 as well as air is which is why there is generally only about 5-6PPM of CO2 dissolved in the water column under standard conditions. If you don't agitate the surface then rate of CO2 diffusion into air is lower as the surface area is less but it is still occurring, for there to be any significant benefit to not agitating the surface then the rate at which the inhabitants are producing CO2 would have to be higher than the rate of diffusion of CO2 into air. But I highly doubt that this would ever be the case. The point I made about an equilibrium earlier relates to whereby the diffusion of CO2 in and out of the water results in a net CO2 transfer of 0 - therefore an equilibrium is present.

 

[FishFanatic04]

so if the concentration of CO2 in your tank is higher than in the air surrounding it then it will diffuse into the air, the rate of diffusion will be faster if you're agitating the surface.

Thanks Jack
i understand this, so i obviously the concentration of co2 in the water is directly related to fish stocking levels & how heavily planted the tank is.
So if you had minimal of both, in theory, agitation would be adding co2 (not that you'd need it in this example)?

I really can't tell you, as it depends on numerous factors such as a the rate of photosynthesis, but in a low tech tank surface agitation isn't an issue anyway as the demands for CO2 from plants shouldn't exceed the concentration of CO2 in the water column.

CO2 in air is about 390PPM, so my previous post needs amending as the concentration gradient doesn't apply itself in the same way as it would if the two bodies were the same ie; air and air. Water isn't able to retain CO2 as well as air is which is why there is generally only about 5-6PPM of CO2 dissolved in the water column under standard conditions. If you don't agitate the surface then rate of CO2 diffusion into air is lower as the surface area is less but it is still occurring, for there to be any significant benefit to not agitating the surface then the rate at which the inhabitants are producing CO2 would have to be higher than the rate of diffusion of CO2 into air. But I highly doubt that this would ever be the case. The point I made about an equilibrium earlier relates to whereby the diffusion of CO2 in and out of the water results in a net CO2 transfer of 0 - therefore an equilibrium is present.

Actually In my low tech, I recently encountered this problem. I dose ferts, but started seeing pinholes and a little melting that looked like co2 issues. I had added a second filter early in the week, and more then doubled the surface agitation. At .65 wpg, I wouldn't expect co2 issues at all.

 

[StandbySetting]

0.65WPG of T5? T8? The WPG rule was based on T12's, personally I consider a low tank to be one that doesn't require fertilisation or CO2 addition, one that is completely self sufficient as far as nutrients go.

 

[ian]

It doesn't matter what wpg you have, C02 issues can arise in any tank...every tank is different. People often forget C02 rise from the substrate as well. The rubbish rotting in the substrate often can give you a nice constant C02 level in a low energy tank.

 

[FishFanatic04]

0.65WPG of T5? T8? The WPG rule was based on T12's, personally I consider a low tank to be one that doesn't require fertilisation or CO2 addition, one that is completely self sufficient as far as nutrients go.

T-8. I don't necessarily Need to dose the ferts. I do it for the same reason I dose my high tech- So there will be an abundance of nutrients and no possibility for deficiencies. I understand your point, but like I said, the extra filter drove out co2. It was also a new tank, so what Ianho said about the substrate wouldn't have applied, there was not much there breaking down yet.

 

[ian]

Yeah that's true, the substrate needs to be more than a few months old.

 

[stu40]

Thanks for the replies..i think

The reason for asking was that i was starting to see co2 related problems (irregular leaf growth on Alternanthera, and a few small holes on other plants)
the only thing i had changed was the spray bar position, it was rippling the surface, where before it was pointed down.

 

[SO19Firearms]

the only thing i had changed was the spray bar position, it was rippling the surface, where before it was pointed down.

In that case it could've been a flow issue in general too, different movement, obstructions and eddies in within the tank. Speed and rate of flow makes a difference too.

IMO