Hello everybody!

A friend of mine recently pointed my attention towards so-called "airlifts" for pond water circulation and filtering.
The principle is very simple, you attatch a vertical tube in your body of water and pump air into the lower end of the tube, witch then bubble upwards inside the tube.
This motion of bubbles propells the water up thru the tube, allowing for a slightly elevated water ejection port, witch can then sift thru a filter down into the pond.

I am no engineer by any standards and i failed highschool physics, but it sparks ideas in my head..

Im thinking; air expanding under lesser pressure and therefore increase speed up thru the tube, hence; longer tube = greater available energy in form of higher water flow upwards.

Now, since not just a part of the water column in the tube is in motion, i dont need a constant stream of small bubbles, i can release large bubbles every x seconds to further increase momentum.

Now, if we were to place a vertical turbine blade in the center of this tube and use the rotational output to power a electrical generator, would it create enough to power a air compressor big enough to feed the system?
If so, would there be surplus energy available?

Ive also discovered that these airlifts are used for water wells, to pump water up from some depth.
This means that the top part of the above suggested system could end several metres above sealevel, and use that fall-heigh to power a oldschool water-wheel.

Now, i come with this idea for the simple reason; please shoot it down for me!
..i need to drop it or build it, but i cant drop it until i know.. :P
..and i dont have the cash for such experiments.. :P

The reason for this idea is mostly to circulate the water in the baltic sea (between sweden, finland, denmark, poland and so on) where the bottom fauna is on some parts dead and dying due to lack of oxygen, while the top living fauna is dying becuase (as i understand it) of lack of nuitrition.
So a generous swap of water would benefit the region for all life, for a very cheap price.

And if there were enough to generate electricity for land-based consumers, that should seriously motivate governments to build their own vertical hydroplants.. :P

So again, please shoot this down for me, so i can drop this idea thats been clinging to my head for too long now..