Backyard Aquaponics
A place to discuss aquaponics

I currently have x6 50mm air stones in my ibc fish tank, I've noticed when the stones sit on the bottom of the tank they put out no where near as much air as if their only 300mm under the surface.

So my question is should I keep them suspended at 300mm or are they actually getting more DO into the water when on the bottom of the tank?


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You will get a longer life from your pump diaphragms (if using that type of pump) if there is less back pressure- ie you dont have your air stones too deep. You will have greater air volume pumped with lower back pressure too, but those air bubbles spend less time submerged. However, I think quite a bit of the Oxygen is added as the bubbles burst at the surface, with the greatly increased water surface area of small droplets exposed to air, and thin bubbles on the surface.

I run my 40lpm pump with the stones about 30cm down, and the ~55lpm at about half a metre- it just delivers air via a poly pipe with lots of small holes drilled in it.

There's a discussion on here somewhere about how the O2 gets added. My view is the model that says most occurs at the surface is not a good on - it sees the bubbles as little balloons travelling through the water, but physics says that cannot be the case.

Thinking at a molecular level the bubbles are interactive volumes; it is extremely unlikely that the molecules which begin the journey at the bottom are the same ones which reach the top, as the interface is not an impermeable membrane but rather closer to a 'zone' of interaction where O2 molecules collide with H2) molecules - in the process some O2 enters the bubble area at the top (because the bubble is rising) and some leaves it at the base.

As the bubble rises it expands in volume because the outside pressure is less - this would increase the interaction area.

The idea that O2 is absorbed at the surface is also problematic - the bubble reaches the surface and 'pops' which means the water on top, held there briefly by surface tension, spatters outwardsand the air of the bubble enters atmosphere not water. There could be a minor effect as the base of the bubble (concave as the bubble bursts) becomes level - the reduction in water surface area might cause some of the air to enter the water volume. Personally I think it is more likely the water will expel the remaining air upwards.

I think you're better off having the air coming into the system from as low as possible; the upward welling of the air will mix higher level DO water into the lower DO water as the bubbles travel - doing that from just under the surface means a lesser amount of mixing will occur. And the longer the bubbles are moving and in contact with water, the more DO you will get.

With the only air coming into the system up near the surface I'd be a bit concerned about low DO areas; maybe watch the fish and see if there are any areas they avoid? My SP and cats hang out near the bottom of the IBC almost constantly, coming up for food obviously, but rarely getting more than 30cms or so off the bottom. I take that as a good sign that there is plenty of DO down there.

My trout mostly hung around near the bottom too, other than when I fed them, and I had no air stones in the bottom half of the FT. There was still a good amount of upwelling water associated with the rising air though, so I reckon the FT water was well mixed, no doubt helped along by the SLO taking water from the bottom and the 3 water returns from the GBs around the top edge.



IMHO I don't think it matters that the air that was in the bubble goes into the atmosphere- lots of tiny droplets of water are formed by the bubble bursting, causing a sudden and drastic increase in surface area of the water that was surrounding the air bubble, a fine mist of water droplets should absorb a lot more O2 than the air-water interface of a rising bubble.

What we need is someone with a logging DO meter to do some experiements! I'll do them if someone will lend me a suitable meter

If you have 6 x 50mm air stones in an IBC it doesn't really matter where you have them, there will be heaps of air whether it be less bubbles down deep or more bubbles up higher...

I wonder if putting the air stones in a pipe that extends to the bottom will help draw low d.o water up while keeping the stones at a shallow depth. Air pump style.

It sure would.

So would I... Can't afford one of my own, but I do have the time. Fluidics is a strange subject.

The SLO would definitely help the circulation - the issue is, does the DO at the bottom come from the inflow from the return flow or from the airstones?

The bubble burst doesn't (I think ) work as you say. The air bubbles do not have a surface. They are a volume formed by the inability of the extant pressure in the water to compress the air further. The bubble does a reverse raindrop shape because of the need to rise versus the pressure trying to prevent it moving.

Water droplets on the other hand DO have a surface - it is formed by the fluid forming the most efficient shape as governed by external forces plus the surface tension. That surface tension is a distinct type of surface - most O2 molecules will NOT penetrate it because of the extra force needed to over come the tension.

Which is why raindrops hit the earth instead of evaporating into mist while still way up there.

Somewhere ( i think in the discussion that JM mentioned ) someone said that when pure oxygen is pumped deep into water using an airstone - i think it was 10 metres - the bubbles dont reach the surface.

Ive never believed that it is most occuring at the top, simply because a finer bubble is preffered and that in itself increases surface area of the tanks gas exchange exponentially as bubble size goes down.

JM, im not up on the physics, but the bubble being put under pressure from the surrounding water, which is what makes it rise?
Gasses under pressure will always flow into the lower pressure?
I would think that the bubbles do exchange DO on the way up.

Sheez, am I the only one that remembers every thread ever created on this fine forum...?


viewtopic.php?f=1&t=12204&hilit=replies

p.s. you may need to prepare your brain... lol

I've read Trout's posts before and with all due respect, Dr Lennard is unconvincing. He hasn't got the figures and his evidence is that Aquaculture pros do it this way - other than that he makes it clear he is guessing. There are many instances when professionals do something a particular way that prove out not to be the best - we normally call it progress and the change often has to be forced from the outside.

Dr Lennard also makes a point that reinforces the views above expressed by me and Yavimaya. He says,
The problem is, he SHOULD be seeing a size difference, even in a metre of water - the bubbles should be noticeably larger due to pressure decrease. What he appears to be confirming here is a significant part of the bubble volume is being lost during the rise - I would hazard that perhaps the O2 is being squeezed out as it tried to push outwards and is being held back by surface pressure - the bubble is restrained from increasing in size so the extra pressure forces air out.

But that's just a supposition based on his statement he doesn't observe any bubble size difference - for some reason he takes that as evidence of little transfer, when in reality I think it shows the opposite.

I'm also unsure about the waterwheel reason - he implies it is solely to do with O2 and DO, but my understanding (which is little enough) is that WW's are used for DO AND current flow. An airstone only provides modest convection so WW's are used where it is needed to have specific water flow.

in the water.

Don't forget to put it on an air pipe

Do people trust me to say that its about 50/50 and very installation specific or do we have to go through it all again?

Yep, there is likely to be some bubble convergence, but i definitely see a size increase in my 1.3m tank.
he also forgets to mention that the more pressure the bubble is put under, the more gas will transfer.
I dont think this issue is put the bed yet, but thats not just here, if there was a clear answer we would know already.