Aquaponics - Aeration

I'm planning on buying some trout in the next few days to grow in my aquaponics system, so I'm thinking a bit about my oxygen levels.

Toward the end of the trout growing period you have a lot of stuff acting against your trout. Your trout are big and make a lot of waste which depletes oxygen as it breaks down, your trout are big and need lots of oxygen, and your weather is getting warmer,  and warmer water doesn't hold as much gas as colder weather does.

You can test this by opening a warm can of beer, or waiting a few years and watching what happens when the oceans stop absorbing carbon dioxide as they warm up a bit.


I have a tube dumping water from my growbed to my sump.

This much water.

It's not under pressure, but is simply overflowing from the growbed at that height. ie this tube's height regulates the depth of the growbed.

no pressure

The tube stopped a few inches above the water and made some nice splashing and bubbling which I thought was adding some oxygen.

I'm probably quite correct in thinking that.

But then I added a tube all the way to the bottom of the tank, and pricked a few holes in the tube just after a narrowing where there was an elbow.

This sucks air into the tube via the Venturi effect.

Apparently, this is because the water experiences higher pressure at the restriction just below the elbow, but then lower pressure when the tube becomes wider again. The result of the low pressure is that it sucks in air.

Which is a little odd.

When you put holes in (or before) the bit that's actually restricted, the holes blow out water.

I know because I checked.

So the result is that oddly enough, air is drawn in and the flow of the water draws it down.

Even more oddly, is that the bubbles don't ever really come out. Some do when the water level is a little low, but these are very small, and not many.

The tube is full of bubbles all in chaos, churning around madly.

It looks like this in real life.

The bubbles enter at the top just where the clear tube starts, and they move all the way to the bottom.

Which is also odd given how gently the water is flowing.

At first I thought they were the same bubbles, but if you block the holes, they clear after a few minutes.


As far as I can tell the air is dissolving.

I have no idea if I'm probably quite correct in thinking this.

If gas really does dissolve in water that readily and in that quantity, it's no wonder fish can live in the stuff.

I think I just got re-interested in the world.

Perhaps it's because I stopped taking all those opiates for my hurty ribs.

120 Things in twenty years  thinks ribs make me less thinky.

[EDIT FROM THE FUTURE - I Thought I'd add a link to wikipedia's entry on Venturi and now that I've read it I don't think I have any idea what I'm talking about, but what I did clearly works. I'm just no longer sure why]


  1. Another thing to note is that surface agitation of the top surface is very good at helping the oxygen level in the water.

    My understanding is that the air-bubble aerator in aquariums do more good by agitating the surface than via the bubbles in the water themselves.

    So maybe a notch on one side so some of the water/bubbles can escape and agitate the surface?

    1. Yeah, that was always my understanding just from talking to the aquarium guy when I was a kid, but the last chat I had with an aquarium guy last week said it was the air.

      I wish someone with a O2 probe and two buckets would test it. There's hundreds of threads all over the world discussing it.

  2. Well, the way lakes, rivers, streams, and oceans do it is via the agitation of the water's surface. However, the level of air content can change depending on the depth of the water. So while you might have excellent surface agitation, without proper circulation, the water at the bottom can become anaerobic. The bubbles in the water help, but not to the degree as the surface does.


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