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Do my air stones need to be at the bottom of my tank?

I have a 1500 gal tank that is 6 ft deep. i just hooked up my sweetwater s21 which will only get air out of the stones when they are only 1/2 way to the bottom. Is it critical that the air stones be near the bottom? I need to know if I have to upgrade my blower or not.

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I would say first that no, your air stones do not HAVE to be near the bottom of your tank. Your fish will probably still survive with the stones half way up the water column. However, the longer the bubbles have to travel to the surface, the more oxygen is exchanged with the water, and the more DO is achieved. Also more degassing is allowed to occur. I would test the DO of your water, and see if it is adequate. If possible, I would also test the DO below your air stones.

I would definitely suggest that you try to upgrade your blower and get your stones down as low as possible however.

I just worked through this in my commercial system (20,000 gallons per system X three systems). We run two S31 regenerative blowers with a third as back up. While installing perferated air lines in our troughs the airstones in the fish tank quit.  When I lifted an airstone to check it, we got air at 2 ft below the surface (the stones are set at 4 ft). So the extra load on the air system by the air lines robbed air going to the airstones. So we then brought the third S31 online and had air again. We are upgrading to a larger blower to meet the demands. We are also going to stagger our airstone depth alternating them at 2 ft and 4 ft deep in the fish tank this will ensure that we have at least 50% of our stones going if we lose a blower. Also, DO is greatly affected by temperature so no matter how much air you pump into the system, you are limited in DO concentrations by barometric pressure and temperature of the water. Here's a link to a DO chart from Aquatic Eco http://www.aquaticeco.com/pages/full_width/151/Aeration-Devices

Thanks for the responses. The problem ended up being the checkvalves I was using. The springs in the valves was causing resistence. As soon as i removed the check valve i was aable to get the airstones down where they need to be. I will be changing tovalves that go onthe intake side of the pump as opposed to the discharge side.

Actually the exchange of air happens in the surface of the water and NOT between the bubble and the water as the bubble goes up. It is the breaking of the bubble at the surface of the water that makes the oxygen go into the water. If you shake the water you will get the same effect. So the air stones can be one inch under the water if you like it. The degassing does not happen so well with air bubbles. You will need a degassing column to do that. 

Yeah, that was my impression of how the water/air exchange occurred.

Angelo Moscariello Basile said:

Actually the exchange of air happens in the surface of the water and NOT between the bubble and the water as the bubble goes up. It is the breaking of the bubble at the surface of the water that makes the oxygen go into the water. If you shake the water you will get the same effect. So the air stones can be one inch under the water if you like it. The degassing does not happen so well with air bubbles. You will need a degassing column to do that. 

Yup at the surface.

And that is likely a myth.  There's a thread on the subject in the BYAP forum.

Angelo Moscariello Basile said:

Actually the exchange of air happens in the surface of the water and NOT between the bubble and the water as the bubble goes up.

 Is it critical that the air stones be near the bottom?

It may not matter, so long as your tank water is being thoroughly exchanged/mixed.  You wouldn't want to have less oxygen at the bottom, for example.  I don't use air stones at all but a lot of splashing takes place in my system and the intake for my pump is at the bottom of the tank and the water rotates, so there is a lot of mixing.  

As George said it may not matter provided the water is being mix thoroughly with whatever other currents are in the tank. The air bubbles will lift some water up however and if it is at the bottom, it will be lifting a current of water from the bottom of the tank upwards causing some better mixing. It isn't actually going to deliver much oxygen to the bottom itself I wouldn't think.

As for where exchange happens, last I checked bubbles have a large surface area. A ton of small bubbles has a ton of surface area so I am sure that doesn't hurt either in addition to the tanks suface. Disrupting the tanks surface as they break is a win win.

I don't know. Now you're making me think. I was so happy in my ignorance until you came along and had to ruin it for me :) I'm finding online sources that say both...Does anyone else knowledgeable on the subject of aeration have any input? (oh, and I was just teasin' George, its a good topic)

George said:

And that is likely a myth.  There's a thread on the subject in the BYAP forum.

Angelo Moscariello Basile said:

Actually the exchange of air happens in the surface of the water and NOT between the bubble and the water as the bubble goes up.

Hi all, When I built a recycling toilet in the 70's we did a lot of work with ultrasonic air foggers and found that to be extremely effective. I still have some of those nozzles and keep thinking of applying them to our AP system. Micro bubbles released at the bottom of the tanks (looks like fog) might be your answer. The ultrasonic tip breaks the bubbles up into billions of bubbles. These nozzles typically come from the commercial oil burner arena. If they were used on home oil burners we could save a lot of oil. They would probably require a diaphragm pump which is what I use anyway as you don't run into the depth cut off problem associated with the turbine pumps you are using.

Also as far as I know it is the surface area of the bubbles and not the bubbles hitting the surface, that counts. Exposure is the issue with air or microbes.

Hey Pete, You may have to make your own zero back pressure check valves. I had to for my water line for the same reason. I did not want to reduce any flow forward at all but quick stop to any back flow. I shared it in a blog. Cost about 6.00 and works great:

http://community.theaquaponicsource.com/profiles/blogs/making-a-zer...

Pete Schuhmann said:

Thanks for the responses. The problem ended up being the checkvalves I was using. The springs in the valves was causing resistence. As soon as i removed the check valve i was aable to get the airstones down where they need to be. I will be changing tovalves that go onthe intake side of the pump as opposed to the discharge side.

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