A tough question on CO2 usage

[gu2high]

When you inject CO2 to water and maintain it at 30ppm, have you considered how much injected CO2 is used by all plants in the tank?

I wonder to reach and maintain a balance of CO2 in the tank, it is likely to be,

injected and solved CO2 + fish/inhabitant released CO2 + absorbed CO2 from air = escaped to air CO2 + absorbed CO2 by plants

If omit the "fish/inhabitant released CO2 + absorbed CO2 from air" as they are relatively small compared to the injected, then,

injected and solved CO2 = escaped to air CO2 + absorbed CO2 by plants

Then, what is the ratio of "escaped to air CO2" to "absorbed CO2 by plants" in a heavily planted and strong lighted and ferted tank? Assume you inject 100 unit of CO2 in the water within one hour, does plant take in 5 unit and remaining 95 unit escaped to air in the period?

[black hills tj]

I'd say it depends on the surface agitation. What is water's ability to retain CO2 over a period of time? If the plants use 5 units in the hour, that does not mean that 95 unites necesarily escaped to the air.

[newfound77951]

A tank with no CO2 injection will have around 3 ppm CO2 in equilibrium with the air. If you are injecting CO2 into a tank, then there is no CO2 absorbed by the air, and the CO2 from the fish is minimal, as you guessed above. The amount of CO2 taken up by the plants is probably a small but not insignificant part of the CO2 injected into the tank. Think of how much O2 is produced when plants are pearling....the O2 coming out equals the CO2 going in.

It would be possible (in a lab) to do an experiment that would quantify how much CO2 is taken up by the plants. Sounds like fun, my lab could actually do it, if I had the time!

[gu2high]

One experiment can be done like this, in a no plant tank, keep all other condition same as what they are, inject CO2 to reach 100ppm, then stop injection. Check CO2 level every 10 or 30 minutes, this may tell us how quickly CO2 escapes to air. I just don't know how quickly it is escaped in a calm surface room temperature tank and is asking about it.

If know the answer above, then adding plant back, do the same experiment, compare the data and this will tell us how fast plant absorbed the CO2 and the ratio of these two.

[theotheragentm]

I would love to see the results of something like that, but who is going to unplant their whole tank? Anybody?

[rkilling1]

Let me add my thoughts on this.

Everyday my CO2 kicks on @ 10:00 and then turns off @ 20:00. I have a drop checker installed with certified 4 KH solution.

When the CO2 turns off at night, the drop checker is green which indicates 30 PPM CO2 saturated in my water column. In the morning when/before the CO2 kicks in, the drop checker is still green.

This means that when I stop adding CO2 in the tank that (1) the plants stop consuming the CO2 and (2) I also do not have any loss of CO2 throughout the night.

The point to this: The amount of CO2 that I inject is what my plants consume. So my plants are consuming 2.5 BPS of CO2 during the time the lights are on.

Points to consider:

My drop checker turns blue when I do PWC's and then returns to green within an hour.

When I lowered my lighting down to 2 WPG, the drop checker turned yellow at the end of the first day. This means my plants were not consuming as much CO2 as before and it was totally expected. I had to lower it to 1 - 1.5 BPS.

I completely dissolve my CO2. So, unlike misting, I do not have any loss of CO2 directly to the atmosphere.

[hashbaz]

rkilling1, so the basic jest of what you are saying is that the vast majority of the CO2 you add is absorbed by the plants. Whould I be correct in assuming you have extremely little surface agitation?

[7Enigma]

Originally Posted by rkilling1

When the CO2 turns off at night, the drop checker is green which indicates 30 PPM CO2 saturated in my water column. In the morning when/before the CO2 kicks in, the drop checker is still green.

This means that when I stop adding CO2 in the tank that (1) the plants stop consuming the CO2 and (2) I also do not have any loss of CO2 throughout the night.

You're missing a key point. Plants respire as well as fish/inverts. During non-lighted periods (and actually even during lighted periods) the plants are releasing CO2. When the lights are on the net flow of O2/CO2 is in the consumption of CO2. When the lights are off the net flow is the production of CO2. In a heavily planted tank when the lights are not on the plants are contributing a significant amount of CO2 into the tank, possibly enough to show on your drop checker.

As for the original question, I can attest to the staying power of dissolved CO2. I did an experiment about 8 months ago at work looking to see just how low I could drop the pH while injecting CO2 (I used dry ice directly in a container of a known pH). While I was only able to get it down around 5 (which is expected since the pKa of carbonic acid falls in that area), I was shocked to see even under heavy agitation with a stir bar the pH VERY slowly went back up. This tells me (as we know) that the capacity in water of CO2 is very high (>1000ppm), but also that it doesn't de-gas from the water as quickly as we tend to believe.

HTH

[rkilling1]

Originally Posted by 7Enigma

You're missing a key point. Plants respire as well as fish/inverts. During non-lighted periods (and actually even during lighted periods) the plants are releasing CO2. When the lights are on the net flow of O2/CO2 is in the consumption of CO2. When the lights are off the net flow is the production of CO2. In a heavily planted tank when the lights are not on the plants are contributing a significant amount of CO2 into the tank, possibly enough to show on your drop checker.

Nope, not missing it. I just didn't add it as it isn't worth considering.

Here is a nice thread on the barr report website:

http://www.barrreport.com/general-pl...ring-dark.html

You can see that what I posted is exactly what they are talking about. And here is a quote from Tom on how much CO2 is produced by plants at night:

"Plants respire very very very little. Fish respire a lot more.
Plants do not move nor have many of the respiratory processes fish do.

So 95% of the respiration is from fish, 3% from bacteria etc, maybe 2% from plants.

If the plants start to rot or die fast?
Then bacteria can consume virtually all the O2.

Regards,
Tom Barr"