Randy can you comment on how co2 dissolution works in this kind of setting, stilled pico fw tank

[brandon429]

Ive been wondering about this for a while now, its a freshwater example but still aquarium science I'd like to know how distribution occurs in the case of a co2 injected but stilled micro aquarium. I'm mainly curious to know how the co2 gets from one end of the tank to the other in what seems like seconds when there are only micro currents and no typical circulation.

This is an API tube array I use to capture pressurized co2 injected into a .5 gallon tank

The tank has a fitted glass lid, its not gasket quality seal but its glass on glass quality. Im sure some air exchange occurs although its fractional to an open topped setup.

The tubes are full of water, and injecting into the lowest tube cascades the gas up to the last tube displacing all the water ready for bright lights.

By around 1 pm the tubes have 10% gas left and are mostly water, this is the repeating cycle of gas in my little tank.


if you capture and find the right tube diameter/bubble surface area that creates a drain rate approximate to your photoperiod you have a completely safe way to gas a micro tank and not kill the neocaridinas breeding within...this array drains from full of gas to full of water in six hours like clockwork so i fill it twice per day off paintball setup and not during the night phase

It took a couple months working up from a two tube array and making incremental guesswork about levels using typical surface seeking behavior of gassed feeder guppies and plant pearling to arrive at the five tube array I'm with, its enough co2 to get anything to carpet now.

[video=youtube;l9PtoXrN6Yk]https://www.youtube.com/watch?v=l9PtoXrN6Yk[/video]

1. If I run the lights without gas there is zero pearling. If I fill the tubes, pearling begins in about one minute on the other side of the tank, yet the tubes don't noticeably decrease the bubble size as that takes hours. Is there some kind of molecular pull and natural distribution system that prevents a bulk of dissolved gas right around the array but instead spreads it evenly across the tank? I've wondered if there is a natural physicality that wants the molecules evenly distributed even without obvious current to aid that. Not sure if thats Henry's law.


2. What is this motion called where the water and gas alternatively fill each tube, I've never known how to term that action.

3. In contemplating partial gas pressure details, consider the actual submersion depth of the tubes. If you take an inverted API tube full of gas and submerge it half way, it has a little tendency to push up with positive buoyancy. If you push it down 8 inches into the water it has a stronger up push, so by extension is that creating a faster drain rate by adding a form of compression to the tubes? Contrast that to the video example above where the water line is low and the tubes are high, thats the lowest partial pressure drain setting im thinking but not sure.

4. The tubes actually don't drain at a measurable difference rate between night and day but common assumptions would tell us the demand is higher for it in the day for obvious reasons. Why does co2 gas out at night at the same rate as the daytime when fixing is biologically restricted? I thought the excess co2 at night from lack of biological uptake would increase partial pressure in the water and draw less out of the tubes. A slower drain rate.

I honestly have that many questions about my tube array lol thanks for setting the gas straight. Here's the in tank run:
[video=youtube;yNx8y-ttJSY]https://www.youtube.com/watch?v=yNx8y-ttJSY[/video]

 

[Randy Holmes-Farley]

This is a very interesting problem with lots of different questions. Thanks for posting it!

I may have to work through them slowly.

Let's start with the very simplest case:

There is absolutely zero bulk water movement (not likely to be real, but an interesting limiting case), and we are just looking at the diffusion of CO2/carbonic acid through the water.

When there is no concentration gradient of any sort, molecules still move around. So, for example, water moves through water. In three dimensions (which means not near an edge of the aquarium), water will move, on average, about 0.001 cm in a second, or about 0.07 cm in an hour at 25 deg C. Some will travel faster and some slower, but that's the RMS (root mean square "average").

So that sort of movement will not really allow many water molecules to move across a tank in a minute, but a few will.

 

[brandon429]

Its fascinating thanks much! Our friends at nano reef.com have also been doing stilled and capped tiny reefs for a couple years now so the micro motion system has some reefing applications as well

My guppies tail makes the largest motion in my tank heh but these guys probably only get Brownian motion transfer and temp stratification transfers if thats known to cause any type of micro motion. Two years is long enough to show the resiliency of these organisms that fare well in little rock depression puddles in the wild when the tide goes out. Imo this mimics their life in the still 'hole in the rock' until water comes back in (his water change time)

Pj reefs has a two year documented model in threads which is quite amazing it has macro, soft coral briareum and live rock with visual benthic growth in a stilled capped container about half a gallon or less and it hasn't gone stagnant and the coral has put on mass
My PJ reef little tank - Pico Reefs - Nano-Reef.com Forums
They do change water as needed but the system has stand alone characters and is a mimic of a stilled rock water depression at tidal flow time. Pj reefs made this a business and has sold hundreds of these for those that didnt want to custom assemble one in any small container. Also lots of copy threads from custom builders using Mason jars etc its really catching on and showing the viability of these organisms

Neat huh

 

[beaslbob]

Dr. Holmes-Farley
On my (beaslbob method? {lol}) Fw planted tanks with no water changes, no mechanical water movement, and a very high bioload, I get an extremely high pH. Like purple (8.4+) on the api test kit.
I presume this reflects a very low CO2 value and the some Fw guys challenge me on that high value.
I was just wondering if there could be a pH gradient with lower pH values deeper in the tanks?

 

[Randy Holmes-Farley]

OK, so a molecule slightly larger than a water molecule (like CO2 or carbonic acid) will diffuse a bit more slowly than a water molecule, but fairly close, so we increase those times by a small amount.

Ions in water do not really feel any "push" to diffuse away from a higher concentration to a lower one (unlike a gas in vacuum where they encounter more pushing bumps on their higher concentration side), there will be net diffusion away from a high concentration to a lower one due to the randomness of diffusion.

When there is some CO2 on the low concentration end of the tank, and a higher amount at the other end, there will be diffusion in both directions, and the fact that some at the low end are randomly moving toward the high end reduces the "net" diffusion of CO2 toward the low end. Ultimately, the net diffusion hits zero when the concentratation is the same on both ends.

But for any intermediate time, there is less "net" diffusion than the pure case for water calculated above, so net overall diffusion is a bit slower again relative to the times I calculated.

As a very rough estimate of the diffusion, we can use this equation:

where n(x,t) is the concentration as a function of the time, t, and distance from your tube, x. erfc is the complimentary error function (a math thing that excel can do), n_o is the concentration at the tube, which is about 3,000 times the normal CO2 level in water exposed to normal air (since you used pure CO2).

D is the diffusion coefficient, about 2 x 10^-9 m²/sec (2 x 10^-5 cm2/sec)


So lets put in some values, such as t = 60 seconds and No = 3000

At x = 0.1 cm we get Co2 concentration = 123 times normal
At x=1 cm we get CO2 concentration = essentially zero molecules

At 1 hour we get

At x = 0.1 cm we get CO2 concentration = 2370 times normal
At x=1 cm we get CO2 concentration = 25 times normal
at x = 10 cm we get CO2 concentration = essentially none


I'm not sure what pearling means, but I'd expect that anything you observe would need a reasonable boost in CO2, and an increase of less than 1% of the total CO2 added would not cause the effect.

At the tank top, the air contains about 0.0004 times as much CO2 as the pure CO2 gas you are adding, so that would seem to be the bottom end of how low the CO2 might get there unless the sealed top actually allows CO2 in the air above the water to become depleted. It could get even lower on the far end of the tank not near the surface, but Ido not know how much lower (pH would tell).

That said, at the tubes you are adding almost 3,000 times as much CO2 as would have been there from equilibrium from normal air.

So in a minute, could 1/3000th of the concentration at one end have reached the other end, effectively doubling the concentration of CO2? No, not by diffusion alone.

 

[Randy Holmes-Farley]

So, I think for you to see anything across the tank in a minute, the CO2 must be moving by convection (water movement) rather than diffusion.

As an interesting test case, you might take a similar tank and add a drop of food coloring and see what happens. My guess is that tiny currents driven by density differences (from temperature differences or salinity differences) are present in the tank, as well as currents caused by fish etc.

 

[beaslbob]

Dr Holmes-Farley
FWIW Pearling is when the plants are so active they give off a string of bubbles rising to the surface. That string looks like a string of pearls hence pearling.
always thought they were oxygen.

 

[brandon429]

Randy thanks I will read that twice and absorb thats excellent. W do a vid of food coloring as well never thought of that!

Bob whats up man
In my opinion there is no gradient unless you are using reducing substrates like Amazonia or something that loses form and compresses over time, a high cec soil which is known to keep pH under control by leaching acids into the water column.
Any bubbles coming out of the plants are certainly oxygen.
Is your tank dirt capped

Where the carbon coming from that is fueling your pearling I'm wondering

Yes I believe your pH could be that high or thereabouts (API?) if you had hard substrate no dirt and aren't running ro di if your plants strip what little co2 there is out of soln
Just my guess.

At saturation point this is the o2 output
[video=youtube;psGQf2rT49w]https://www.youtube.com/watch?v=psGQf2rT49w[/video]


Randy thanks for the fw discuss it sent me on a new reading path.

 

[beaslbob]

Brandon
not much man except PH LOL
While I do get an initial lower ph (7 or less) with peat moss in the substrate, eventually even that rises to over 8 in a few weeks. the same as with crushed coral, sand and other substrates.
by contrast, more "normal" (expected) ph values are measured with jars I kept in darkness with no plants.
And the pearling does seem much more active initially before the ph rises. but still have some even then.
I presume the fish are providing the carbon. One 10g tank had 30 guppies with 1/2 dozen reproducing adults for instance.