Hypothetical Co2 Blanket?

I have a 30 gallon tank heavily planted with CO2 injection. I have a canister filter and the return pipe is an inch or so below water surface. I also run an air stone at the other end of the tank mainly for water circulation.
The CO2 runs on a timer to coincide with my lighting period of 7 hours. I am using a diffuser rather than a reactor so whilst the majority of the CO2 does get dissolved some of the bubbles reach the water surface and are ''lost'' to the atmosphere.
My tank has a hood and also has condensation trays which sit about an inch above the water surface. Obviously they do not form an airtight seal but between the condensation trays and the hood they must reduce the amount of air exchange between the water surface and the room to almost nothing, even with the window open and a breeze coming into the room virtually nothing will happen in the space between the water surface and the condensation trays. The layer of air between water surface and condensation trays will be virtually stagnant, the only interruption to this is from the air stone drawing fresh air from the room via the air pump.

I don't claim any expertise in these matters but I do remember from my schoolboy physics lessons that carbon dioxide is heavier than air ........so......... is it not reasonable to assume that any CO2 bubbles that reach the water surface and pop will just deposit a layer of CO2 over the water surface due to it being heavier than the air? Obviously this will not happen all the time the air pump is running and disrupting the airspace with clean air delivered by the air stone but what would happen if I removed it? Would the CO2 bubbles which pop at the surface cause the entire water surface to be covered in a blanket of CO2?

I don't intend to try it, this is more of a hypothetical discussion point - it would only become relevant if the air pump packed up.

 

I would not run an airstone while CO2 is running, you are stepping on your own feet that way.

Start CO2 2 hours before lights come on, and shut off 30 minutes before lights shut off.

Point your canister return so it is rippling the surface.

Run the airstone at night when the CO2 is off only.

There will not be a blanket of CO2 at the surface, even if there was it would't dissolve into the water, it doesn't have a "reason" to... (no forced pressure).

BTW, atmospheric CO2 levels are 400+ ppm, and we are trying to get roughly 30ppm in our water... so a blanket of CO2 won't achieve that if 400+ppm air won't raise water CO2 concentrations.

 

I would not run an airstone while CO2 is running, you are stepping on your own feet that way.

Start CO2 2 hours before lights come on, and shut off 30 minutes before lights shut off.

Point your canister return so it is rippling the surface.

Run the airstone at night when the CO2 is off only.

There will not be a blanket of CO2 at the surface, even if there was it would't dissolve into the water, it doesn't have a "reason" to... (no forced pressure).

BTW, atmospheric CO2 levels are 400+ ppm, and we are trying to get roughly 30ppm in our water... so a blanket of CO2 won't achieve that if 400+ppm air won't raise water CO2 concentrations.

Kind of misleading. If there were a co2 blanket, it would exchange into the water with the ripples produced from the filter or air stone.

Also, atmospheric ppm and our tanks ppm are measured differently. They aren't really apples to apples but you're right that water holds less gas than... well gas I guess.

You don't have a co2 blanket however. Any movement on the surface of the water causes some air movement, which will easily move the co2 around, which would mix with atmospheric air. The only way to really get this effect would be to have dry ice sublimed co2 fall into the open areas of the tank.

 

Good points above. I may add few, also going back to my schoolgirl physics lessons.

Yes, CO2 gas is heavier than air. Therefore, at least temporarily, the lower layer of air that is in contact with water will have a higher concentration of CO2. And Henry's law is telling us that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid.

On the other hand, the air under the hood has a higher temperature due to lighting. Another physics law states that as the temperature increases, solubility of a gas in a liquid decreases.

I don't know what to make between those two laws but I don't believe it makes a difference in the amount of extra CO2 dissolved.

 

Thanks for the replies, as I said it was just a hypothetical question and even then I didn't really put it across very well. I was thinking more about the oxygen levels in the water rather than CO2.
In the event that there was no surface agitation and the excess CO2 could form a thin layer over the entire surface of the tank how would that affect oxygen levels. Oxygen gets into the water by gas exchange at the water surface, if the entire surface was covered by a thin layer of CO2 presumably it would prevent any oxygen getting into the water.

 

Thanks for the replies, as I said it was just a hypothetical question and even then I didn't really put it across very well. I was thinking more about the oxygen levels in the water rather than CO2.
In the event that there was no surface agitation and the excess CO2 could form a thin layer over the entire surface of the tank how would that affect oxygen levels. Oxygen gets into the water by gas exchange at the water surface, if the entire surface was covered by a thin layer of CO2 presumably it would prevent any oxygen getting into the water.

Well, yeah. If the tank was essentially sealed with co2, and you were pumping co2 into the water, there wouldn't be any oxygen. Your fish would die in the day, and whatever was left would die in the night. Plants wouldn't do well at night either.

 

Thanks for the replies, as I said it was just a hypothetical question and even then I didn't really put it across very well. I was thinking more about the oxygen levels in the water rather than CO2.
In the event that there was no surface agitation and the excess CO2 could form a thin layer over the entire surface of the tank how would that affect oxygen levels. Oxygen gets into the water by gas exchange at the water surface, if the entire surface was covered by a thin layer of CO2 presumably it would prevent any oxygen getting into the water.

"Oxygen gets into the water by gas exchange at the water surface". That's true but if you have healthy plants, a lot more oxygen is produced by the plants than provided from the air.

 

Not when the lights go out! That's when then major die offs typically occur in co2 injected tanks!

 

"Oxygen gets into the water by gas exchange at the water surface". That's true but if you have healthy plants, a lot more oxygen is produced by the plants than provided from the air.

Yes, plants produce oxygen as a by-product of photosynthesis but that only occurs in daylight. When they stop photosynthesising not only do they stop produce oxygen but they actually start to consume it.

 

When lights are off, plants are giving off some of the CO2 they consumed during photoperiod and consume some of the oxygen they produced during that time. However, overall, plants are taking in CO2 and give off oxygen.

In my CO2 injected tank I have a glass lid that seals off pretty tight any contact with air. To make things worse, my Frogbit often covers 80-90% of the water surface. Undissolved CO2 bubbles accumulate under the Frogbit leaves. I don't run an airstone at night and the water surface is as still as it can be. And I am yet to see fish gasping.

 

When lights are off, plants are giving off some of the CO2 they consumed during photoperiod and consume some of the oxygen they produced during that time. However, overall, plants are taking in CO2 and give off oxygen.

In my CO2 injected tank I have a glass lid that seals off pretty tight any contact with air. To make things worse, my Frogbit often covers 80-90% of the water surface. Undissolved CO2 bubbles accumulate under the Frogbit leaves. I don't run an airstone at night and the water surface is as still as it can be. And I am yet to see fish gasping.

When is your pH drop from using CO2?

 

When is your pH drop from using CO2?

Did you mean to ask what my Ph drop is or are you asking when it occurs. As luck would have it I slightly upped my CO2 recently and tested it over the last 2 days.
My photo-period is 7 hours I tested before the lights came on (CO2 is on the same timer as the lights) Ph before CO2 starts is 7.17 and immediately after it stops it reads 6.14 then returns to 7.1 again overnight.

 

Did you mean to ask what my Ph drop is or are you asking when it occurs. As luck would have it I slightly upped my CO2 recently and tested it over the last 2 days.
My photo-period is 7 hours I tested before the lights came on (CO2 is on the same timer as the lights) Ph before CO2 starts is 7.17 and immediately after it stops it reads 6.14 then returns to 7.1 again overnight.

a 1.0 drop is a good level. You can probably bump it up just a little more if you want. If you do that, ensure you have good surface agitation so as to have good O2 levels as well as CO2 levels.

 

a 1.0 drop is a good level. You can probably bump it up just a little more if you want. If you do that, ensure you have good surface agitation so as to have good O2 levels as well as CO2 levels.

I've only just started using Ph drop to monitor CO2 levels, I have always used a drop tester before but was never completely satisfied, the change of colour from green, yellow and blue is somewhat dependent on how good the light is when you check it.
Even using Ph drop is open to interpretation, a 1.0 drop is good by all accounts but that would really depend on your starting point. The Ph scale is logarithmic so a drop of 1.0 from 7 to 6 is going to be considerably different to a drop from 6 to 5. I did read a suggestion that you can go as far as dropping by 1.4 at which point the fish will start gasping at the surface, just back it off a tad at that point. I'm not brave enough to try that, it almost seems reckless.

 

Half of my rainbow fish keep to the top of the tank right around co2 shutoff time. The rest of them act normal. I added a mj1200 for more agitation so we will see how that goes

 

I've only just started using Ph drop to monitor CO2 levels, I have always used a drop tester before but was never completely satisfied, the change of colour from green, yellow and blue is somewhat dependent on how good the light is when you check it.
Even using Ph drop is open to interpretation, a 1.0 drop is good by all accounts but that would really depend on your starting point. The Ph scale is logarithmic so a drop of 1.0 from 7 to 6 is going to be considerably different to a drop from 6 to 5. I did read a suggestion that you can go as far as dropping by 1.4 at which point the fish will start gasping at the surface, just back it off a tad at that point. I'm not brave enough to try that, it almost seems reckless.

Regardless of the pH starting point (kH) the CO2 drop method is not reliant on the log scale of pH. a 1.0 drop equates to roughly 30ppm of CO2 - 8.0 down to 7.0 or 6.0 down to 5.0. I have noticed high pH tanks can drop a bit lower... say 1.6.

 

Regardless of the pH starting point (kH) the CO2 drop method is not reliant on the log scale of pH. a 1.0 drop equates to roughly 30ppm of CO2 - 8.0 down to 7.0 or 6.0 down to 5.0. I have noticed high pH tanks can drop a bit lower... say 1.6.

True if KH is 4 dKh. If your KH is say...20 or more, it will take a lot of CO2 to drop pH by one unit.
I don't measure KH or pH but I have a drop checker. I've seen it yellow several times, usually toward the end of CO2 canister (when pressure drops to about 700 psi). Last time, I waited too long (canister pressure about 500 psi). My angelfish was floating on its side. I was about to reach for my Clove oil. I did a water change and the fish recovered instantly. My friend is telling me that you see that often with the lake fish...they are floating on their sides, just like they are drunk but once startled, they swim away.

 

CO2 affects pH the same regardless of kH.

CO2ppm = 3 x dkH x 10^(7.0 - Tank pH)

ex: kH of 4, pH of 7.6 and a pH drop from CO2 of 1.0:

3 x 4 x 10 ^ (7.0 - 6.6) = 12 x 10^0.4 = 30ppm CO2

Now, same scenario but with: kH of 12, pH of 8.0 and a pH drop from CO2 of 1.0:

3 x 12 x 10 ^ (7.0 - 7.0) = 36 x 10^0.0 = 36ppm CO2

So, regardless of the kH of the tank water, and assuming 3ppm ambient CO2 concentrations in water @ equilibrium with the air, a 1.0 drop in pH from CO2 will yield roughly 30ppm CO2.

 

CO2 affects pH the same regardless of kH.

CO2ppm = 3 x dkH x 10^(7.0 - Tank pH)

ex: kH of 4, pH of 7.6 and a pH drop from CO2 of 1.0:

3 x 4 x 10 ^ (7.0 - 6.6) = 12 x 10^0.4 = 30ppm CO2

Now, same scenario but with: kH of 12, pH of 8.0 and a pH drop from CO2 of 1.0:

3 x 12 x 10 ^ (7.0 - 7.0) = 36 x 10^0.0 = 36ppm CO2

So, regardless of the kH of the tank water, and assuming 3ppm ambient CO2 concentrations in water @ equilibrium with the air, a 1.0 drop in pH from CO2 will yield roughly 30ppm CO2.

Nope.

 

CO2 affects pH the same regardless of kH.

CO2ppm = 3 x dkH x 10^(7.0 - Tank pH)

ex: kH of 4, pH of 7.6 and a pH drop from CO2 of 1.0:

3 x 4 x 10 ^ (7.0 - 6.6) = 12 x 10^0.4 = 30ppm CO2

Now, same scenario but with: kH of 12, pH of 8.0 and a pH drop from CO2 of 1.0:

3 x 12 x 10 ^ (7.0 - 7.0) = 36 x 10^0.0 = 36ppm CO2

So, regardless of the kH of the tank water, and assuming 3ppm ambient CO2 concentrations in water @ equilibrium with the air, a 1.0 drop in pH from CO2 will yield roughly 30ppm CO2.

See the link for a chart on the relationship between pH, KH, and ideal CO2 ranges. Achieving 30ppm is entirely dependent on KH at a given pH (edited for clarity).

 

Nope.

And why is that?

See the link for a chart on the relationship between pH, KH, and ideal CO2 ranges. 30ppm would be deadly at some pH levels and insufficient at others, depending on the KH.

kH / pH / CO2 charts don't work for accurately measuring CO2 in tank water because they assume only carbonates and only CO2 are influencing pH.

 

And why is that?



kH / pH / CO2 charts don't work for accurately measuring CO2 in tank water because they assume only carbonates and only CO2 are influencing pH.

It still disproves that a 1.0 drop in pH will always yield 30ppm +/- of CO2.

 

It still disproves that a 1.0 drop in pH will always yield 30ppm +/- of CO2.

 

Huh... I stand corrected.

 

Chanyi

I don't get your math...You are saying that the amount of CO2 can be calculated by three times the dKH multiplied by 10 at the power of 7 minus tank pH value?

In the first example (KH 4), is 12 x 10 at the power of 0.4. This is 12 x 2.5=30. Fine.
In the second example (KH 12) you changed the starting pH to 8 and get 7 minus 7=0....Any number at the power of zero is 1. Correct but why did you change pH?
The second example should be the same drop of pH from 7.6 to 6.6 if KH is 12.
And the answer is 3 x 12 x 2.5 =90 ppm. Which prove that for the same drop in pH (of one unit in your case), you have more CO2 if KH is higher. Or in other words: you need to inject more CO2 to lower the pH by the same amount if you have a good buffering.

 

Chanyi
In the second example (KH 12) you changed the starting pH to 8 and get 7 minus 7=0....Any number at the power of zero is 1. Correct but why did you change pH?
The second example should be the same drop of pH from 7.6 to 6.6 if KH is 12.
And the answer is 3 x 12 x 2.5 =90 ppm. Which prove that for the same drop in pH (of one unit in your case), you have more CO2 if KH is higher. Or in other words: you need to inject more CO2 to lower the pH by the same amount if you have a good buffering.

I changed pH in the second example to show another example. Starting pH of 8.0 with a 1.0 drop from CO2 = 7.0.

If kH is 12, pH would not be 7.6 (would be more like 8.0 or even higher) which is why I used those numbers. If your water has a kH of 12 degrees and only a pH of 7.6 then something is out of the normal, and you shouldn't apply this formula for that case.

If we use the formula and apply a kH of 12 and a pH of 7.6 dropping to 6.6 we get:

= 3 x 12 x 10^ (7.0 - 6.6)
= 36 x 10^ 0.4
= 90 ppm which you got as an answer as well. This is proving that something else in the tank is buffering the water, and should be addressed or taken into account. A typical tank with a kH of 12 will have a higher pH than 7.6. If not, something is acidifying the water and buffering it to 7.6.

Lets use my actual tap water as an example:

kH 15 degrees, pH 8.2 dropping 1.0 point from CO2:

3 x 15 x 10^ (7.0 - 7.2)
45 x 10^ -0.2
= 28ppm CO2.

A 1.0 drop still = roughly 30pm CO2.

 

Sure there are other buffers and/or acidifying agents in the tank water, so let's take the fresh rain water and check your formula.
KH is 0. pH is 5.6 and CO2 about 350 ppm.
Let's give KH a bit of value (or else we'll end up with zero), say 0.01. We will also make the drop in pH from 7 (pure water) down to only 6, so we have one unit of pH change.

3 x 0.01 x 10^ (7.0- 6.0)
0.03 x 10
= 0.3 ppm CO2

I believe that rain water with 200 ppm CO2 will have a pH of 5.9 (close to 6).

Where am I wrong?

 

1) Not my formula** I am simply passing along information from extremely experienced, and smart planted tankers.
2) These are fish tanks, not scientific case studies with control (rain water) experiments.
3) No matter what, there is no completely accurate way, at the hobbyist level, to accurately measure CO2.
4) Guidelines (this formula, the kH/pH/CO2 chart, pH drop measurements etc.) are guidelines only, not foolproof textbook precision laws.
5) If you have to deliberately input fixed numbers to yield a desired result, it does not help anybody determine how much CO2 to inject into their planted tank.

Current most common and widely accepted method to express CO2 levels in a planted tank is using pH drop. Anything from 1.0 - 1.4 is considered "safe" obviously starting on the lower end watching livestock health.

Remember, surface agitation, water temp and livestock species are important in determining how much CO2 you can dissolve.