Guyute:
yes, the bottom set of leaves keeps the plant from going into the water, that's what holds 'em up
as far as seeds/seedlings, I've never tried to grow from seed in my aero setup, it's basically a 'cloning machine' for me...my suggestion if you want to go all the way from seed to harvest in it would be to start the plants in perlite, then when they have enough leaf matter to support themselves in the tubing, put 'em in...better yet, grow a crop in soil/hydro and put the cuttings in aero...
the air pump I use is designed for 10 to 30 gallon fish tanks...I think the more air you can do, the better...maybe...
changing water, I just take off the entire top and set it on something (like another container, so the roots just dangle), dump it out, fill it back up, and put it back together.
spray jets aren't needed in this system. when I was originally thinking about this, I wanted to use spray jets, but then the cost goes WAY up, so does the maintainence. With spray jets, you need a submersible water pump (minimum $20), and the spray jets clog up all the damned time (especially if you have hard water).
vulpina, here's a simple side view..
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heh...pretty simple, eh?