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Evaporation and CO2 stability

Themuleous

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Joined
6 Jul 2007
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4,121
Location
Aston, Oxfordshire
Simple question really;

Could evaporation cause a CO2 instability?

Before I put a cover on it (partly to test this question), my 40lt used to lose c.5lt a week and has been atrocious at growing plants since I set it up a few months back. Could the loss of what amounts to 12.5% of the water over the course of the week cause the CO2 levels to be unstable and thereby affect the growth of the plants?

Sam
 
Yes, Barr mentions it often. I found that on my 60cm if I let the water level fluctuate too much I could induce algae seemingly at will.

You're essentially creating fluctuating levels of co2, I found turning the heater off made effects much less because the evaporation was reduced. Adding a cover will increase levels of co2 because less will off gas but this also mean that 02 levels will be reduced. Are the problems now under control?
 
I wonder if it is more to do with the CO2 that must build up (for tanks using CO2 injection) in the area between the water surface and the cover, which I can only assume must in part make its way back into the water in the same way that oxygen does?
 
I think it might be that it's as an over all level, if you're adding 25ml to 100ml it's going to be less diluted if the water volume then drops by 20ml.
 
Think you answered your own question here :)

bcoldwine said:
the fluctuation was large enough to upset the light co2 balance.

Ok the with lower water the CO2 should go up, but as far as I remember it is really the stability of the co2 rather than the specific amount. No one is ever going to achieve exactly 30ppm after all. Also, and I should have said this, but as the water level in the tank drops so it gets closer to the outlet and therefore surface water agitation goes up driving off CO2.

What was my thought process behind the observation anyway :)

Sam
 
Hi All

If the return water pipe whether it be a spray bar or lily pipe is situated high or sub surface of the water level and then taking into account the rate of evaporation / poor observation of the water level dropping would induce more water agitation which would vent the residual gas within the water column.

Sam’s statement of

Themuleous said:
Could evaporation cause a CO2 instability?

Does sound feasible - evaporation can / or cause a CO2 instability but from the point of causing BBA I doubt very much – but if the plant keeper is not observant of drop checkers changing colour algae could be induced.

The question to be asked is? How long would it take to induce an outbreak of algae through the lack of injected Co2.

Regards
paul.
 
I think its something like - when the CO2 concentration increases, the plants automatically gear themselves for higher metabolism. If then the concentration drops, the plants can't cope or something. I think someone mentioned this before as another reason why non-CO2 injected tanks should not have regular water changes in order to keep the CO2 stable.
 
Sam, whatever causes water to evaporate also causes CO2 to evaporate even more quickly. At higher temperatures gas is less soluble in water so gas will evaporate faster than water causing CO2 concentration and O2 concentration levels to drop.

In an open top tank there is greater surface area exposed to atmosphere and so more of everything evaporates compared to a covered tank. This is the same issue faced by tanks with uncovered sumps which tend to de-gas CO2 more quickly and also tend to evaporate water more quickly.

Various combinations of water loss and out-gassing can lead to either lower than desired general CO2 levels or can amplify instability. The problems manifests itself to a greater or lesser extent depending on the other factors in your technique because all mistakes combined are cumulative.

Cheers,
 
bcoldwine said:
I'd love to hear an explanation of how this occurs.

Garuf said:
Yes, Barr mentions it often. I found that on my 60cm if I let the water level fluctuate too much I could induce algae seemingly at will.

I always thought the idea with co2 was to provide a slight excess in relation to lighting, same priciple as EI? Or rather keep levels at maximum within the limits of livestock comfort then light accordingly. So there shouldnt be a problem with any slight fluctuation. Is this over simplifying the issue?
I really cant see how a fluctuating water level could induce algae on its own unless the fluctuation was large enough to upset the light co2 balance. And surely with lower water levels co2 levels rise with a constant supply. I'm forever fiddling with co2, doing random water changes and topping up, I have never had bba. I appreciate your not talking about bba specifically, I'm just using it as an example of what occurs in a fluctuating co2 environment, or did I get that wrong?
Im looking forward to the answer to your question.
I believe the reason is fluctuations in levels of co2, the algae I could induce was thread/dust, if I paid attention and topped up every day it was fine and would clear up with in a few days if I ignored topping up and only dosed it'd show it's face around the 3rd day. I marked the tank and filled to the same level every single time and not a sign at all.

I don't know if the increase in hardness from topping up has any effect but it might be something to consider.
 
Thread and dust are typically due to low general CO2 levels. CO2 Instability typically triggers BBA. If you're experiencing filamentous/dust then it probably indicates that lighting is on the highish side or that injection rate/flow/distribution is on the lowish side.

Cheers,
bcoldwine said:
I always thought the idea with co2 was to provide a slight excess in relation to lighting, same priciple as EI? Or rather keep levels at maximum within the limits of livestock comfort then light accordingly.
No, this is not entirely true, although you are correct in that slight fluctuations don't matter. The principles of nutrient application have nothing to do with the principles of CO2 application. However, the level of CO2 that you add is directly related to the level of nutrients that you must add, so the two are connected that way but not in the manner in which they need to be added.

As flyga mentions, plants adapt physiologically to the level of CO2 in the water. If the level changes it costs them energy to make a new adaptation.

Think of it this way; have you ever travelled somewhere and suffered jet lag? It takes a few days to adapt to the new time zone. Then you travel home and have jet lag at home for a few days. Imagine if you woke up in a different city every day. Would you ever feel "right"? probably not. This might be what a plant feels like when faced with unstable CO2.

bcoldwine said:
I'm forever fiddling with co2, doing random water changes and topping up, I have never had bba. I appreciate your not talking about bba specifically, I'm just using it as an example of what occurs in a fluctuating co2 environment, or did I get that wrong?
Ironically, fiddling with CO2 and changing water may actually stabilize CO2. Tap water is very high in CO2. You can tell exactly how much CO2 is in your tap water by drawing some tap in a cup and measuring the pH. Then let the cup sit for an hour or so and then measuring the pH again. Use each of the two pH values in the pH/KH/CO2 formula to calculate CO2.

As plants grow, their nutritional and CO2 needs increase, while CO2 availability decreases due to blocked flow, cylinder/regulator anomalies and so forth, so that fiddling makes some corrections to availability. It's the relatively large shifts in CO2 that trigger BBA.

Cheers,
 
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