o2 vs co2

[silvershark]

ok so you get air pumps to "inject" oxygen into the tank by creating water movement at the top of the tank as the water at the top is oxygen rich, thus mixing it up.

AND you get co2 pumps to increase the carbon dioxide levels for healthy plant growth.

isn't one gonna counter act the other? and also i heard the there are specific times of the day the each should be used as plants absorb oxygen during the night, is this true?

 

[cybergoldfish]

Plants do absorb oxygen during the night/when the lights are switched off. I'm not sure if the oxygen they produce during the day makes up for this, or if it poses a danger for the fish in the tank due to lack of O2, but putting an airstone into your tank at night would mean having to put considerably more CO2 in in the morning. Any plant experts here? Maybe you should ask in the plant forum.

 

[Def]

ok so you get air pumps to "inject" oxygen into the tank by creating water movement at the top of the tank as the water at the top is oxygen rich, thus mixing it up.

By rippling the surface of the water, you are creating gaseous exchange allowing the tank water to absorb more oxygen. Venturi systems (the ones that put huge amounts of bubbles into the water) increase the contact time between o2 and water, increasing o2 absorption.

AND you get co2 pumps to increase the carbon dioxide levels for healthy plant growth. isn't one gonna counter act the other? and also i heard the there are specific times of the day the each should be used as plants absorb oxygen during the night, is this true?

Plants benefit from CO2 during the day, as they synthesise it into oxygen. If you search for "pearling" you'll see lots of posts where planted tanks visibly create oxygen.

Running CO2 at night is a risk. Personally when I had CO2 injection it caused huge problems. I nearly lost a lot of fish due to lack of O2 in the water when CO2 was running overnight. That was with a powerful filter and venturi nozzle.

 

[silvershark]

so is a co2 injector useful for a heavily planted tank?

 

[nc_nutcase]

I was told that plants remove the need for a bubble maker in the water. That was in a discussion about a tank that had a pretty strong filter which rippled the top continuous.

I'm not declaring that is a solid fact... but that's what I was told when I had a similar conversation with a few planted tank geeks... I’m interested in hearing (well, seeing) the rest of the dialog here…

 

[Ed4567]

Before I comment I have little experience on planted tanks so take this with a pinch of salt..

nc_nutcase I would have thought that through photosynthesis plants would introduce oxygen into the water during the day reducing the need to the water surface to be agitated. However at night when plants are not photosynthesising at night they continue to respirate reducing the concentraion of oxygen in the water and increasing the concentration of carbon dioxide increasing the need to gas exchange at the surface of the water.

Perhaps someone with a planted tank could clarify this, when adding carbon dioxide to the tank is the purpose to increase the concentration to an unnaturally high level or just to keep up with the carbon dioxide removed by the plants?

 

[Dorkhedeos]

well, if you let water sit, you cant dissolve anymore oxygen into it.

 

[Yenko]

so is a co2 injector useful for a heavily planted tank?

Yes. The levels of CO2 and O2 do not affect each other. Common ways of oxygenating the water drive off CO2, but that is because water disturbance adds oxygen and removes CO2. Generally a planted tank with a moderate fish load does not need any oxygenation. Personally I don't think it is wise to stock a fish only tank where you need anything more than a bit of water movement from the filter to keep the water oxygenated.

The water movement is almost nonexistent in my 10 gallon tank, but the water is so well oxygenated that a goldfish (Which generally requires cooler water because of the higher oxygen levels possible) does not gasp at the surface, even in the mornings. I have no way to test for oxygen but the bubbles of pure O2 on my plants probably increase the tank's O2 supply substantially. In addition, when the bubbles do rise to the surface they stay there without popping, so they keep enriching my water with oxygen.

 

[silvershark]

can a heavily planted tank do without a co2 injector then?

 

[Bignose]

The main thing to remember is that the water-air system is always striving for equilibrium. If we assume that the air is constant (your tank is not going to significantly affect the composition of air in the room/building) air is 78% nitrgoen, 21% oxygen, and CO2 in the air is 0.033%.

If you give water and air system enough time, this will be the percentages you find in the water, too. What is enough time? well, if you just sit out water and dont agitate it or impose movement in it, it could be days. However, with the currents caused by filters, airstones, and even the fish themselves the tank can be considered to we pretty well mixed, which means that gas exchange will depend solely upon the surface.

If the surface remains undisturbed (filter output underneath the water level, no airstone) the rate of gas exchange is slow, but bubbles and filter returns increase the agitation which significantly increases the gas exchange.

However, whatever the rates of gas exchange is, the system is always striving toward equilibrium. What I mean is that, in your tanks, you have a sink of oxygen, namely your fish and the plants at night. That means oxygen from the air will always be trying to get into the water.

Similarly, you have a source of CO2, the fish again, and the plants at night. During the day, there is both a source (fish) and a sink (plants) of CO2. So, depending upon the fish and the plants, there may be CO2 leaving the water or CO2 entering the water at the water-air interface. CO2 is not necessarily driven off at the surface, nor will any other gas, it all depends on where your tank is as compared to equilibrium. Most likely, however, the CO2 levels in the tank are higher than 0.033%

Typically, plants can be a very strong sink of CO2, which is why people construct the CO2 injectors. Now, this suppplements the CO2 from the fish with CO2 from another source. In this way, compared to equilibrium, the CO2 in the water is over equilibrium, so at the air-water interface, CO2 will leave. This is why those with CO2 injectors typically try to minimize surface distrubances, to that the rate that the CO2 leaves is lowered. And, when the lights go off, the plants are now one of the three sources of CO2 (fish, injector, plants) and with the two sinks (fish, plants) and minimal surface disturbance, the O2 levels may get too low to support life.

In a similar manner, this why extra agitation is recommended when medicating -- many of the medicines are oxidizers, they attack the protozoa or bacteria by taking oxygen from the water and attaching it to the molecules in the infective creature. In short, the working medicine acts as a sink of oxygen, which again will pull the system out of equilibrium. The increased agitation will help increase the rate at which the system reaches equilibrium.

There are more complicated issues, like the solubility of CO2 is like 20-30 times the solubility of O2 in water, but ultimatly, if you just consider that the system will always be striving toward equilibrium and the greater the air-water surface disruption, the quicker the system will hed toward equillibrium, you can get a long way in analyzing and understanding your tank.

p.s. if you would like to see a fairly refreshing read on how to successfully keep a planted tank, Diane Walstad's Ecology of the Planted Aquarium is very informative. She does not believe that CO2 injectors and a whole bunch of fancy equipment is needed. Just some basic sense.

 

[steelhealr]

The main thing to remember is that the water-air system is always striving for equilibrium. If we assume that the air is constant (your tank is not going to significantly affect the composition of air in the room/building) air is 78% nitrgoen, 21% oxygen, and CO2 in the air is 0.033%.

If you give water and air system enough time, this will be the percentages you find in the water, too. .

Hi..this is not totally true. We all have to go back to physics and remember gas kinetics. We have to think of molecules in motion and think of them colliding with each other, bouncing off and creating pressure. Do we all remember something called the gas law? PV=nRT? The partial pressure of a gas is equal to the sum of it's dissolved components and Henry's law that the partial pressure of oxygen in water is directly proportional to the amount dissolved in air. However, cold water holds more oxygen than warm water; aquarium water will hold less oxygen on a day with 100% humidity compared to 20% humidity...water vapor will reduce the partial pressure of oxygen in the air and on a day with higher barometric pressure, more O2 will dissolve (remember the carbonation in soda example).

By dissolving CO2 in the water, you may effectively be reducing the partial pressure of oxygen and thus decrease O2 (Pt=p1 + p2 + p3, that is the total pressue of a dissolved gas is equal to the sum of it's dissolved gases). All the posts above are correct. When photosynthesis ceases for the day, plants return to aerobic respiration and will absorb O2 and give off CO2 at night. Just a little insight..science class adjourned (bell rings). SH

 

[nc_nutcase]

I swear I understood all of that

One thing I did pick up on in reading it...

However, cold water holds more oxygen than warm water; aquarium water will hold less oxygen on a day with 100% humidity compared to 20% humidity...

So most of us have a top on our tanks that hold in a lot of the water that evaporates off the aquarium. Therefore this barrier is going to be very warm (same temp as water) humid air... which is about the worse condition for the gas exchange right?

So would I understand that simple aeration of the water surface will draw us closer to 'equilibrium'?

 

[steelhealr]

Hi..I think that this the nice advantage of an outside power filter. The intake tube pulls the water up from the bottom of the tank, exposes it in the media chamber and then tumbles it out thru the cutout that is NOT under the hood, agitates the water at the surface, and increases water surface area exposed to air. etc. I don't think anyone would want a very tight seal over the top of their tank. SH

 

[Bignose]

Sure, the equilibrium point will be shifted by pressure, humidity, temperature, pH, etc. etc. But my main point was that the system will always head towards equilbrium. If there is more CO2 in the water than the equilibrium, CO2 will be lost at the surface to the ambient air; if there is more O2 in the air than the equilbrium, O2 will be taken into the water at the inteface. The rate the system reaches equilibrium is most strongly a function of the surface agitation.



For steelhealr, I really did not want to get too much into the partial pressures and all that. Solution and VLE (vapor liquid equilibrium) gets really hard really fast if you want to do it all right. Henry's law is just for a perfect system or at infinite dilution, almost no real systems obey that. Same goes for Raoult's law, and the perfect gas law. Then, most VLE systems thoeries are for binary mixtures, this would be at least a trinary: water, O2, and CO2. Not ot mention the trace impurities. Or a component that is not an impurity at all, but the concentrations of N2 may have an affect. So, really 4 species in the VLE system now. Then, the CO2 can interact and lower the pH of the system, so the physics of the problem should take into account the ionic strength of the solution as well. Which would be a function of all the ions that make up the hardness of the water as well as anything else ionic at all. That is, all the calcium, magnesium, nitrate etc. etc. ions. And finally, all the reactions must be described in some way. This becomes a very difficult problem. But, the one sure thing that the system will always be driven toward equilibrium, whatever that may be.

 

[nc_nutcase]

I know there are members here from all over the world, but let's stick to English please...

So... is 'equilibrium' a suitable point for both plants and fish to thrive?

If yes, then creating a flow of fresh air at the water/air interface as well as having adequate agitation at the waters surface would assist the water at reaching and lingering near 'equilibrium', would this be correct?

If 'equilibrium' is less than suitable, then we all gots a whole lotta work to do

I’ve had a repetitive thought for many years about the air barrier at the water surface. It’s hot humid sometimes smelly air. I knew that barrier layer had to have an influence on the water and therefore the fish. I commonly leave the doors to the aquariums open during the day with the thought that ‘I’m giving my fish fresh air’, but until now I would have never admitted this as… fish don’t need air… But now thanks to Doctor BigNose and Professor Steelhealr… well… I cannot say I learned anything as the conversation started slightly over my head and rose quickly. But I tried to follow along, and got something out of it.

I’m being sarcastic here guys. I don’t mind being the dumb guy in a conversation with well educated people (do it a lot actually, don‘t say it!). It’s good to know hear this level of mechanics and it’s even better that there are two of you. Almost anyone can develop half a theory and make up a bunch of big words to make it sound right…but since there are two educated individuals having a friendly debate on the topic, we get to watch you guys keep each other in check… kind of like watching our fish have territory battles… lol