Air Stones....

[snazy]

Regarding the second point there, nature always tends toward equilibirum. The only way water would achieve a level greater than 100% saturation is if conditions change -- such as the water warms up quickly. Given constant conditions, nature won't overshoot equilbirum and then have to come back down. Outgassing would only occur if conditions significantly changed from a previous state.

Super saturation of oxygen in water does happen in nature at certain times and conditions. There is another source of oxygen in water, besides diffusion law and water movement, and this is called photosynthesis.

Then you would think wrong. Because the laws of diffusion do not change depending on the medium it is diffusing through. Diffusion through a gas, liquid, or a solid are identical. Now, the specific diffusion rates will be different, but the laws remain identical, and hence the mathematical forms of the solution remain identical.

Do you really claim that diffusion of gas in water and diffusion of gas in air happens at the same rate?
When you use a mathematical formula that contains non-constant values which change depending on the conditions, such as these in a diffusion law, the solution is hardly identical.

 

[Bignose]

Do you really claim that diffusion of gas in water and diffusion of gas in air happens at the same rate?
When you use a mathematical formula that contains non-constant values which change depending on the conditions, such as these in a diffusion law, the solution is hardly identical.

Are you even reading what I am writing...

Oxygen diffusion through nitrogen (in effect air) has a diffusion rate of around 0.250 cm[sup]2[/sup]/s
Oxygen diffusion through water has a diffusion rate around 0.000025 cm[sup]2[/sup]/s.

About 4 orders of magnitude difference (the specific values will be a function of temperature, in both cases), as is typical when comparing diffusion rates in liquids and gases.

The law is the same. The description of phenomena is the same. Diffusion is described by dc/dt = D d[sup]2[/sup]c/dx[sup]2[/sup] in gases, liquids, and solids. If you non-dimensionalize the equation, the non-dimensional solution is the same, because the laws of diffusion are the same across a very wide width of phenomena. Obviously, the coefficients are different, but the laws are the same.

ok, if we look at a real world example, life in the african rift valley lakes is limited to the upper reaches of the water as there is not enough oxygen to support life (the deeper water is anoxic). There is no real current to move the water around, only diffusion to distribute dissolved gases. These lakes have been there for millions of years yet diffusion has not completely oxygenated the water?

A large body of water isn't really all that comparable to a fish tank. Large bodies of water get all sorts of run-off, a lot of which contains chemicals like pesticides and herbicides and fertilizers and pollution -- a lot of which is heavier than water and also consumes oxygen. Not only that, but decaying matter that would settle on the bottom of the water also consumes oxygen. Both of these do a good job limiting the amount of oxygen that water can hold. Hopefully, your fishtanks aren't like that.

 

[Bignose]

P.S. I have never claimed that a certain number of bubbles will oxygenate the water. You came up with formulas and bubble numbers where all I was trying to say is that regardless of whether bubbles dissolve in water or not, the bubbles do hit the surface causing water surface movement and thus helping in oxygenating the water.

And, I've always agreed that bubbles help increase the mixing, which is a good thing. But, you can't argue about the mixing benefits of bubbles without also mentioning the mixing benefits of a filter return. A filter return does a fair amount of mixing without an airstone. Hence, the mixing doesn't really have much to do with the question of whether more oxygen comes in from the tank top or from the bubbles.

Now you are argueing that diffusion alone without water mixing will still reach all levels. Yes, I agree, but how many thousands of years will that take? It certainly hasn't happened in the Black Sea yet.

The incredibly well validated laws of diffusion say so -- again, why such disbelief in laws that have literally been validated trillions of times over? Again, a large natural body of water is a poor comparison to a fishtank. Much more going on in a natural body of water than we have in a fish tank.

 

[snazy]

It's not that I don't believe in diffusion law, on the contrary. However, if diffusion alone is able to provide enough oxygen within the required time frame, then it invalidates the need of water surface movement in a fish tank, and not only an air pump is not needed in this case, but also filter spray bar, power heads, etc... Which I don't think is quite right.

And I also don't deny the benefits of the filter return flow and water circulation. I just said that the efficiency depends on how it is setup.

Anyway, thanks for sharing your views. It is indeed appreciated, although it may have not always sounded this way. I think we started going in circles

 

[Bignose]

It's not that I don't believe in diffusion law, on the contrary. However, if diffusion alone is able to provide enough oxygen within the required time frame, then it invalidates the need of water surface movement in a fish tank, and not only an air pump is not needed in this case, but also filter spray bar, power heads, etc... Which I don't think is quite right.

There are planted tanks out there that are very successful with no mechanical water movement at all. Diana Walstad has many examples. Obivously during the day, the plant photosynthesis provided additional oxygen, but she didn't run a filter at night when plants will actually consume oxygen. I don't have much of an idea of how much oxygen the plants provide or consume, but I do think that it is another example of a tank with enough oxygen where the oxygen gets around the water mostly from diffusion and a little bit of mixing from the movement of the fish.

 

[Lufbramatt]

So maybe airstones are not useful because of oxygen diffusing from the bubbles themselves, but due to the water movement they create, pushing water up that is near the air stone and across the top of the tank where it becomes exposed to the atmosphere? Point understood about diffusion, but then it depends wether the rate of diffusion can keep up with the rate that the oxygen is being consumed by the tanks occupants.

 

[snazy]

There are planted tanks out there that are very successful with no mechanical water movement at all. Diana Walstad has many examples. Obivously during the day, the plant photosynthesis provided additional oxygen, but she didn't run a filter at night when plants will actually consume oxygen. I don't have much of an idea of how much oxygen the plants provide or consume, but I do think that it is another example of a tank with enough oxygen where the oxygen gets around the water mostly from diffusion and a little bit of mixing from the movement of the fish.

Exactly, plants grow from the bottom up, producing the oxygen needed at the bottom layers that otherwise will not be there in a non-planted tank with no pumps. From what I have read, the maximum amount of oxygen levels in nature is reached in the evening, because of photosynthesis.
However, in a CO2 injected tank, when you create nearly artificial conditions to get all kind of plants that can't be seen in nature together to grow so lush in a tiny amount of water, with the help of high amount of CO2 that again is not necessarily seen in nature at such a constant rate, these CO2 concentrations may reach dangerous levels at night, so running the air pump on will drive this out of the tank.

 

[Bignose]

So maybe airstones are not useful because of oxygen diffusing from the bubbles themselves, but due to the water movement they create, pushing water up that is near the air stone and across the top of the tank where it becomes exposed to the atmosphere? .

Yes, I have posted pretty much exactly this several times now. I have always agreed that the mixing caused by the movement of the water entrained by the bubbles is a good thing.