filters, powerheads, and oxygen exchange

[godzilla]

I asked this question on another forum but figured this might be a better place to ask.

i have a question about my setup.

currently I am running an eheim 2217 and an AC110 filter on my 55, as well as a small powerhead near the top that has the hose attached, which creates bubbles (adjustable, set very low now)

I am going to replace my AC110 with another 2217, simply because I want silence in my family room, where the tank resides.

I have my spraybar for the 2217 pointing at the back of the tank, facing towards the bottom, and my fish LOVE playing in the current. if I do the same with the other, I would have very little surface agitation.

My question is, will the powerhead be enough? is it surface agitation I am after or are the bubbles that are introduced via the powerhead enough to invoke the O2 exchange I need? I can point the 2nd spraybar up for surface agitiation if I must, but I was just curious...

FYI its a 55g tank with 20 demasoni, 6 yellow labs, and a few pleco's - so the bioload is not low.

[andywg]

You want surface agitation rather than bubbles. Bubbles do not tend to stay in the water long enough to allow any significant gas exchange. So long as you aren't monstrously heavily stocked I would expect a powerhead pointing towards teh surface should be ok.

[TomK2]

Bubbles can provide awesom gas exchange. In effect, they dramatically increase the surface area of the tank. Each little bubble is a tiny sphere, whose surface area is.... uh...recalling my geometry from years ago.... something like 4 Pi (R squared). When the bubble hits the surface, it adds its surface area to the tank as it pops. Sure the bubbles are tiny, but there are a lot of them! Thus, bubbles improve gas exchange like surface agitation does (another way to increase surface area).

It has been demonstrated that you could use Carbon dioxide bubbles instead of air, and the oxygen levels would rise! Thus, it is the bubble surface area effect, not what is in the bubble, that improves gas exchange. That is why CO2 diffusers strive to keep the CO2 bubble in contact with the water as long as possible, it takes a bunch of time for the gas in the bubble to exchange with the water. Heavily planted tanks with CO2 injection strive to minimize gas exchange so that the CO2 stays in the water.

[toddnbecka]

Most of the gas exchange takes place at the surface of the water. As long as there is good circulation in the tank (4-5 times the tank volume/hour is fine if not overstocked) you shouldn't have any problems.

[andywg]

Originally Posted by TomK2

Bubbles can provide awesom gas exchange. In effect, they dramatically increase the surface area of the tank. Each little bubble is a tiny sphere, whose surface area is.... uh...recalling my geometry from years ago.... something like 4 Pi (R squared). When the bubble hits the surface, it adds its surface area to the tank as it pops. Sure the bubbles are tiny, but there are a lot of them! Thus, bubbles improve gas exchange like surface agitation does (another way to increase surface area).

It's not so much that the bubbles are tiny or have a high surface area, but they just aren't in the water long enough to allow any significant or appreciable amount of gas to dissolve from the bubble to the water.

While they will increase surface agitation upon reaching the water's surface, IME they cause nowhere near as much surface agitation as a powerhead or filter outlet pointing at/across the surface.

[7Enigma]

Originally Posted by andywg

It's not so much that the bubbles are tiny or have a high surface area, but they just aren't in the water long enough to allow any significant or appreciable amount of gas to dissolve from the bubble to the water.

While they will increase surface agitation upon reaching the water's surface, IME they cause nowhere near as much surface agitation as a powerhead or filter outlet pointing at/across the surface.

They most definately do have enough time. Gas exchange is passive diffusion, both into and out of the bubble. The smaller the bubble the larger surface area, the more/quicker gas exchange occurs. When you use a PH you are finely chopping up the bubbles so that they have extremely high surface area to volume ratio. This allows for very good exchange.

The PH as used in the OP's example is essentially an air pump. If there was no motor to chop up the bubbles it might not be very effective since you'll just be getting big burps of bubbles. When they hit the surface they will allow for gas exchange from the turbulence caused by the bubble breaching the surface.

Once the PH is turned on though you are dealing with a whole different animal. Now you have surface area air contact IN the water column as well as when they reach the surface. Your also creating an underwater current which increases the turnover of water at the surface and substrate levels.

OP, a very important factor in making sure enough O2 is present in the water is making sure the top of the tank is not completely sealed. This is extremely important for setups that do not use an air pump/PH, but still somewhat in your case. Since you have a canister filter and a HOB there is that area where the waterfall goes into the tank. If you remove the HOB I would recommend keeping at least some of that gap present to allow for large-scale gas exchange. But with your setup, you will be completely fine.

HTH

[TomK2]

The gas exchange is not between the bubble and the tank water coming out of a bubble stone to any appreciable extent. The bubble hits the surface, increases the surface area of the tank-atmosphere interface as well as agitates it (just another way of saying increases surface area), and gas is exchanged between the atmosphere and the tank water. What is in the bubble, be it nktrogen, carbond dioxide, air, or oxygen is irrevelant.

Look at a CO2 diffuser for a heavily planted tank. They are often racks that the bubble must slowly climb before it exits the top. This is so that the bubble will be in contact with the water for a great lenght of time and then have time to exchange with the water, a process that takes quite a while. Much longer than the trip a bubble makes leaving a bubble stone and travelling to the surface.

Consider this. Take a tank with a low oxygen level and high CO2 level. You hook up a carbon dioxide canister to an bubble stone. You are now bubbling pure CO2 in fine bubbles from the bottom of the tank to the surface. You will find that oxygen levels rise and CO2 level will fall into equilibrium with the atmosphere. Thus, no gas exchange between the bubble and the water, but the increased surface area at the tanks surface speeds up CO2 diffussion out of the tank and oxygen diffusion into the tank.

Think of surface agitation and surface area as the same thing. The rippling water, since it is not flat, has greater surface area than still water and gas exchange is enhanced. It does help that the gasses that have been exchanged are carried away and spread through the tank by circulation too, this gets the gasses evenly distrubuted.