lots of talk about aquarium pumps lately... is there a recommended output / gallon???

[Magnum Man]

assume it has more to do with depth, & amount of accessories the pump is hooked up to... when I 1st started getting back into tanks, I bought this pump...

Amazon.com

www.amazon.com

it's rated for between 20 - 200 gallons in the 1st place that's IMO to big of range of tanks... I'm sure that's just a "sales" pitch

not sure how that rating was come up with... but it works fine for my 55 gallon, that's not in my fish room, driving 2 - 10" air bars... I would say it's not big enough, if I had wanted to drive 4 air bars in the same 55 gallon tank...

makes me wonder... if there is a CFM / accessory recommendation somewhere... I'm not really very "normal" if something doesn't meet my expectations, I either make it an emergency back-up, or put it on a tank it would work on, or junk it, & chalk it up as a learning experience... but if an average guy, just getting into aquariums bought a pump for a tank, that had several accessories, & it wouldn't drive it, it might not be possible whether for budget issues, or may not have a 2nd tank to put it on???

 

[Wills]

I think for air pumps like that unless its driving a sponge filter (or similar) the rating does not matter too much - all air stones do is provide surface agitation to allow gas exchange between the air and the water, they are mainly decorative IMO.

 

[Magnum Man]

I know it's not directly pumping O2 into the water... but I think sometimes they get under rated... every little bubble has a surface area for oxy absorption into the water... I don't necessarily think a fine mist adds more oxygen... IMO, it would be the surface area of the bubbles... so if a pump is putting out a set amount of air, I have no idea if 1000 tiny bubbles has more actual surface area than 100 bigger bubbles ( discounting the oxygenation from the surface agitation ) I would suspect they would be equal ??? but one may cause more surface disruption... of course time exposure would effect the oxy exchange... so I think you would get a better exchange from the air introduction at the bottom of the tank, than would from the same stone 1" below the surface, even if the surface agitation were equal

 

[Uberhoust]

The smaller bubbles have a significantly larger surface area than the equivalent volume of larger diameter bubble. Assuming a sphere (bubbles really don't form a sphere) a 1 liter bubble volume would have s surface area of 438.5 cm2, and a radius of 6.2 cm. A sphere of 1 ml volume would have a surface area of 4.38 cm2, but you would have a 1000 of them resulting in a total surface area of 4835 cm2 (vs. 438.5 for the same volume as a single bubble). This relationship is fixed primarily because surface area is based on the radius squared and volume is based on the radius cubed.

More gas exchange surface area will mostly affect how closely the dissolved gases in the water relate to the partial pressures of the same gases in the air. More gas exchange can result in less oxygen in the water when there are a lot of photosynthesizing plants growing in the tank, or more oxygen in the water during the night when photosynthesis is not occurring.

This is why CO2 diffusers try to generate microscopic sized bubbles, one they move slower in the water column and two they have much more surface area per unit of volume than larger bubbles.

 

[Colin_T]

Aquarium air pumps come in 1, 2 and 4 outlets. The single outlet pumps and some twin outlet pumps can come in a range of sizes, whereas the 4 outlet pumps usually have the same size internal components (diaphragm & air chamber) as the big twin outlet pumps.

As a general rule, if people have a small aquarium that is about 12 inches high and want to run 1 or 2 things (air operated filter and an ornament), then a small single outlet air pump is normally sufficient.

If they have a tank that is 18 inches high and want a couple of air operated items, then a big single outlet pump or a twin outlet pump is better.

If they have several tanks 18 inches high, or a tank that is 24 inches high, then a big twin outlet or a 4 outlet air pump is the better option.

 

[FranciscoB]

"I don't necessarily think a fine mist adds more oxygen... IMO, it would be the surface area of the bubbles... so if a pump is putting out a set amount of air, I have no idea if 1000 tiny bubbles has more actual surface area than 100 bigger bubbles ( discounting the oxygenation from the surface agitation ) I would suspect they would be equal ??? "

Interesting thread -
The main effect of aireation with a pump+air stone is not to inject Oxygen into the tank. In fact, what is being delivered by the pump is air (not oxygen, thus a mixture of whatever is in the air the pump draws from). The air is not "injected" into the water in the sense that the bubbles rise to the surface almost completely unchanged from being released from the airline, all the way to the surface air/water interface; thus, the bubbles themselves don't cause much gas exchange while traveling in the water column. Also, their popping (bursting) at the surface is releasing that air mostly to the air above the tank, not to the water in the tank. Instead, is the convective current of water from the bottom of the tank, that is moved by the bubbles up to the surface, with the resulting opposite travel of surface water back to the bottom what results in significant gas exchange (O2 and CO2 being the most significant in this system). That exchange occurs mainly at the surface, and almost not at all throughout the water column. Thus, the constant convection currents moving water from bottom to surface is what causes the most gas exchange. A water pump or other device positioned correctly would accomplish the same, with no air involved.
Now, about the size of the bubbles - There is something important there, but not related to the delivery of Oxygen, but to the relative ability to move water by streams of differently-sized bubbles. Both large and small bubbles will themselves release nearly nil (almost zero) oxygen (or air) while traveling in the water column; there is just not sufficient contact time, and there is major surface tension to be impeding such exchange within each bubble and the surrounding water. However, it has been shown experimentally that steady and rich injection of small bubbles moves more water convectively than the same amount of air delivered in much larger bubbles. So, generally, smaller bubbles are better, so long as there is enough air flow to produce many small bubbles (very few, small bubbles will not create much of a convection current, and will also not release any significant amount of either oxygen or oxygen-rich air into the water column). So, there is a sweet spot of best (optimum) air delivery speed and bubble size that needs to be tailored to specific aquarium heights.
And then, our preferences play all kinds of roles, customs set-ups and other reasons why people like the look this way or that, all of which are add ons onto the basic principle behind.

 

[TwoTankAmin]

First, it is true that finer bubble beat larger ones for aeration.

Does the total surface area of two submerged air bubbles remain the same if they merge?

"No. Volume scales with the third power of the radius, while surface area scales with the square of the radius. Merging two bubbles will create a bigger bubble, but the surface area will be less than the sum of the two original bubbles."

If you want to see the math and the explanation -->

https://www.reddit.com/r/askscience/comments/q5a2ki

Next, in most tanks this won't matter what size bubbles may be because the other factor involved is contact time. The bubbles rise to the surface and burst before almost any gasses can become dissolved in the water. What does help regulate gas concentration in tanks is the gas exchange which happens as a result of the surface being roiled. The surface tension of an ummoving water surface inhibits the exchange of gasses between the water and the air. The action of the bubbles at the surface breaks the tension.

This is why in high tech planted tanks where CO2 in added there needs to be special action/equipment to get it dissolved in the water.

I use some larger diaphragm air pumps in my smaller fish spaces- not enough to be called a fish room. I have 2 that handle 8-15 outlets and one that does 12-20. I also have better quality manifolds for these. I do use smaller pumps to fill some smaller needs. Aside from being the driving force behind foam filters, they provid lots of oxtgenation because they roil the surface. I even use a few airstones for extra agitation.

 

[Back in the fold]

Always get a bigger one than you need . You can bleed off the excess air through a tee and the diaphragm will last longer . JEHMCO sells heavy duty diaphragm air pumps as well as the linear piston type and they are worth a look . Whatever you do keep a spare diaphragm on hand because you will need it sooner or later . That’s another one of those skills every aquarist should have - the ability to change out an air diaphragm. Very easy to do .

 

[Uberhoust]

the bubbles themselves don't cause much gas exchange while traveling in the water column

The intent here is not to critique @FranciscoB, but I have heard this statement before starting back in the late 60s with my father telling me essentially the same thing, ie the bubbles really don't play a significant role in the gas exchange other than by causing water motion. I don't necessarily agree with this so decided to run some numbers to see if it makes sense.

Using a typical 75 gallon tank as an example with the following assumptions:

1. Bubbles rise at approximately 30cm/s (for our purposes we likely should just time them, but I don't have the tank in front of me, this is a complex area of study. Value determined by personal conversation with my son who studied water treatments)
2. Internal pressure in a bubble is 14 psi (1 atmosphere) - I know this is wrong, smaller bubbles tend to have higher internal pressures due to hydrogen bonding on the water surrounding a small unit of gas and the bubbles start out closer to 15 to 16 psi just due to the depth of water. I do not know the effect on gas exchange but I suspect it is minimal.
3. There is constant cycling of the water in the tank, this can be driven by the bubbles or by other sources, ei filter, powerhead etc.
4. Bubbles are spheres - Valid for small bubbles but larger ones are definitely not, but a sphere is the most conservative estimate in this case.

5. Dynamics of an initially spherical bubble rising in quiescent liquid | Nature Communications

6. 
   

Surface area of tank 5574 cm^2 (this is in constant exposure to the air)
Assuming 1 mm bubble diameter each bubble is 0.000524 ml and you would have 60000 of them with a litre of air at 1 atm.
Assuming a pump that pumps 4 litres a min and a bubble exposure time to the water of 1.5 seconds you end up with a surface area exposure of the air to water during a minute of 6133 cm^2 which is slightly more than with the surface area of the tank alone.

Conclusion the bubbles themselves are important to the overall gas exchange, providing that is your goal, and finer bubbles provide the largest exchange. I can share the calculations with anyone who is interested.

 

[Uberhoust]

FYI I just guessed about 4 litre/min. The rating for a tetra whisper 100, what I use, the volume per minute is closer to 3.3 litres/min. This reduces the value of surface area exposure to 5060 cm2 which is under the surface area exposure. If using larger bubbles ie 2mm the exposure is now closer to 2500 cm^2.

So if gas exchange is important aeration will only significantly help if the bubble size is under 2mm but more ideally if they are under 1 mm.

 

[Magnum Man]

Breaking out of silence for a minute ( sorry I couldn’t contain myself )….. like like like

That was my thought all along… and if your pump is of sufficient size to do a surface area of the tank every 10 seconds ( just for example ) the bubbles alone equal a significant amount of surface area…

Also, as the bubbles rise, actual surface measurements are gong to get bigger, as the bubbles expand, as they rise, due to reduced pressure against them…

Sorry, reverting to my other thread, so many things I m this hobby are passed along as facts… and depending on the type of person you are, facts are bits of information, just waiting to be disproven… back to my little corner

 

[Uberhoust]

In my brief foray into bubble sizes the most interesting thing I found was that the surface tension makes very small bubbles under significantly higher pressure than one would expect. When I get home I will be looking at this further... For CO2 injection they use quite small bubbles when they are using a diffuser. Found a bubble of 0.01 cm diameter has 50% higher internal pressure than one would expect. Not the size of the bubbles we are talking about. A 0.006cm bubble has double the expected pressure.

 

[FranciscoB]

The amount of oxygen released by bubbles into the water is still negligible when compared to the gas exchange caused by the surface agitation and convective motion of the total volume of water. In addition, the bubbles are not oxygen, but air.