Can someone explain simply please. CO2, aeration, KH and low tech tanks.

[hamfist]

Hi, I believe aeration and KH can both affect availability of CO2 to plants in low tech tanks and have read a number of threads and sources about it. However, I am still trying to fit it all together into a "coherant whole" in my head. Surely there must be a simple explanation.

So, to maximise CO2 levels in non-CO2 injected tanks during the photoperiod, should one ........

• reduce surface agitation ?
• reduce or stop aeration ?
• aim for any "ideal" KH range ?

I think ideally I should do the first 2, and have a "significant" KH level (maybe something like 5-8 dKH ?)

Obviously livestock needs also have to come into the equation and almost everything will be a compromise in the end, but if we ignore livestock, whats the situation purely for CO2 levels ?

Many thanks, Alan

 

[Andy Pierce]

In a low tech tank KH has no effect on dissolved CO2. Surface agitation and aeration do the same thing: they speed up how long it takes for CO2 levels (and oxygen levels as well) to stabilise with respect to atmosphere - aeration is much more effective at this than surface agitation. Unless you have something actively adding CO2 to the water (e.g. livestock but I suspect this is actually a pretty limited source) it doesn't matter whether you agitate/aerate or not. Aeration is nice because it keeps oxygen levels up for livestock, gives you some water column movement, disrupts biofilm and can 'look nice' depending on your perspectives on this type of thing. For sure your plants will be consuming CO2 during the photoperiod so if you want that replenished (from atmospheric CO2) you will need to agitate/aerate.

In a low-tech tank the ideal KH range is the same thing as the ideal pH range - they move together. What is 'ideal' depends on what you're trying to do, e.g. if you want caridina shrimp, KH needs to be 1 dKH or lower. Many people (including me) find most plant growth challenging at high KH e.g. 15+ dKH, but there are still some species that can be successful (alternanthera, crypts, quadricostatus, java fern, java moss etc.).

 

[dw1305]

Hi all,
Edit: What @Andy Pierce says in the <"previous post">.

So, to maximise CO2 levels in non-CO2 injected tanks during the photoperiod, should one ........

• reduce surface agitation ?
• reduce or stop aeration ?

Other way around. If you are low tech the larger the gas exchange surface area is the more gas exchange you will have. More surface agitation means that more CO2 will diffuse into the tank water along the concentration gradient between water and atmosphere. If you don't have surface movement you can end up in the situation we had at <"Canford Park">.

but it was a pond and the water sample had a dissolved oxygen level of 180% (~20oC, 18mg/L DO) and a pH value of pH 10.5.

The high pH value is dependent on the amount of dissolved CO2, and we know that the <"equilibrium pH value in carbonate buffered water is ~pH8">, so we have very depleted CO2 levels (and conversely very raised oxygen levels).

• aim for any "ideal" KH range ?

I think ideally I should do the first 2, and have a "significant" KH level (maybe something like 5-8 dKH ?)

This is a bit of a strange one, you need @hax47 or @Andy Pierce for the scientific explanation, but you have more Total Dissolved Inorganic Carbon (T(D)IC) in carbonate buffered water <"CO2 relationship to KH">, but it is less plant available for plants without adaptation to take up bicarbonate (HCO3-).

cheers Darrel

 

[ElleDee]

There are some conflicting schools of thought about this, so I'll just give my current thinking.

If you have a substrate with organic material (aquasoil, soil), CO2 from bacterial respiration decomposing the organic matter can greatly surpass atmospheric CO2. This decomposition requires oxygen, so I aim for a fair amount of gas exchange, but have never gone as far as adding extra aeration.

The "ideal" range for kh is as low as possible. I see no benefit to adding it unless your livestock requires it. Some people with ro water will add a half to 1 dkh, but others don't add any. I get 1.5 dkh out of the tap myself.

 

[MichaelJ]

Keeping water temperature lower will sequester more CO2 as well...

... why the warming of the oceans is a double whammy when it comes to atmospheric CO2 levels.

Cheers,
Michael

 

[hamfist]

Wonderful info guys. You have confirmed that what I was initially doing was right - ample aeration & surface agitation. I have recently read a load of rather confusing stuff and so had recently been questioning my original decisions. Its good to now get it straighter in my head.
Over the last year or two I've been having a lot of success with my smaller planted shrimp tanks with plenty of aeration, so I knew i could make it work like that. Its just that my latest project (500 litre + significant fish load) is another "kettle of fish" (completely inadvertant pun usage there).
WIth KH I have been manipulating it according to livestock needs, and having an RO filter that has been reasonably easy. I don't really want to go the CO2 route, so I am trying to get absolutely everything "right" in an attempt to avoid going down that rabbit hole.

Thanks again to all who responded !

 

[hax47]

Other way around. If you are low tech the larger the gas exchange surface area is the more gas exchange you will have. More surface agitation means that more CO2 will diffuse into the tank water along the concentration gradient between water and atmosphere. If you don't have surface movement you can end up in the situation we had at <"Canford Park">.

I would expect that in most low-tech aquariums, the CO2 level is still higher than the equilibrium CO2 level with that of the air-CO2. That means that if the aeration/surface agitation is increased, the CO2 will usually escape from the aquarium. This is dependent on the air-CO2, which, in my room, is low during the day when I am not home. When I am home and I sleep in my room during the night, the air-CO2 levels can raise above the equilibrium levels in the water. In this case, CO2 can be indeed dissolved into the water from air with aeration (check the red curve when it is above the blue in the below plot - than CO2 diffusion can happen from air-to water). But this period is rather short and happens during the night. Actual CO2 levels and dynamics in other homes could be totally different from mine though.

Production of CO2 in the aquarium requires oxygen, so decreasing aeration could be a double edge sword. To little aeration could lead to too low oxygen levels.

This is a CO2 recording from a low-tech tank, parallel measurements of CO2 in air and water. The y-axes are aligned, so the concentrations can be directly compared, CO2 will diffuse from higher values to lower values. Note that the aeration led to a massive CO2 drop in this measurement. Otherwise, the air CO2 is almost always below the aquarium levels (actually the partial pressure is, which is not shown directly, but is proportional to the concentrations).

 

[dw1305]

Hi all,

You have confirmed that what I was initially doing was right - ample aeration & surface agitation.

I just think it is the safest route.

If you have a substrate with organic material (aquasoil, soil), CO2 from bacterial respiration decomposing the organic matter can greatly surpass atmospheric CO2. This decomposition requires oxygen, so I aim for a fair amount of gas exchange

That was Diana Walstad's initial premise <"Interesting fact about natural CO2 in Lowtech/Walstad">, and also some of her rationale <"for a lighting siesta">.

Because I'm <"obsessed with"> (paranoid about?) dissolved oxygen, I don't want to take any gambles with DO levels and I'm willing to trade lower levels of CO2 for "oxygen security".

cheers Darrel

 

[dw1305]

Hi all,

Production of CO2 in the aquarium requires oxygen, so decreasing aeration could be a double edge sword. To little aeration could lead to too low oxygen levels.

That is always my worry, that the Biochemical Oxygen Demand (BOD) system will exceed the oxygen supply, with catastrophic results. Because I've had the <"good fortune"> / misfortune to work with some very polluted <"water"> I'm probably overly cautious.

In wastewater treatment you ideally want to outgas any CO2 as efficiently as possible, partially due to its pH depressing effect. Because of the dangers of CO2 build up in confined spaces traditionally you just vented it to the atmosphere, which isn't ideal <"A Critical Decade: removing greenhouse gases from wastewater treatment | Water UK">.

cheers Darrel