After further thought, I was considering maybe growing in long sphagnum moss (not powdered peat) to help with the pH, which can spike to the low 8s in a drought, and to set another example for the kids as it's a biodegradable growing medium unlike the sponge I considered. Do you think that would work well and not clog the system up? However, getting the solid waste out of it would be impossible without wrecking the whole grow bed. I'd need to pre-filter the water in that case. Which, I just realized, is possible with a simple and cheap sponge filter setup.
> If stocking density / feed application / nutrient application rate is too low you could add other nutrients to system for driving plant growth.
I have considered that. On the one hand, I'd like to keep it as much self-sustaining as possible; it would be a better example of a balanced ecosystem for the kids to not add any nutrients but fish food. But on the other hand, as an aquarist, I'd much rather set it in their impressionable minds that 20 2" long fish in a tank is not the norm - not without at least 50% water changes 2x/week, heavily planted either immersed or emersed, lots of aerations, and a large power filter! And potentially with no backup system, I'm afraid to, what a strict aquarist considers, massively overstock the tank. The whole tank would go belly-up over a single weekend if the power went out, maybe hours.
Yeah, I may have to just go with additional nutrients. I can't stop thinking of the kids going home and asking their parents for a 10 gallon tank with 20 goldfish. Or 20 anything!
> Head I am concerned about is the distance between water level of fish culture tank and where it exits lift tube to enter the root medium. I use air lifts and they can be very demanding with respect to air volume needed to move a given amount of water once head gets above an inch or so. I said inch. Test you airlift now to see how high it can move water with your existing air-supply. You should be able to rapidly determine the relationship between airlift tube length above water and resulting water flow if all other aspects are kept constant. It is a frequent problem for airlifts to come up short with respect to water flow they can deliver. Having intake for air-lift low is good but keep it away from return of standpipe to prevent short circuiting of flow.
Ah, I see. Well, I wanted to keep the water at least a couple inches below the bottom of the grow bed (jumping fish bonking their heads) and also have the top of the lift tube an inch above that, same as the standpipe, so the roots would stay in an inch of water no matter what. That's 3 inches minimum.
I do think I have one pump that could do that. However, the AirPod I mentioned is much more powerful, and I'm sure it could lift the water that far. I do have a 10 gal with a UGF that I can use for a test when I get down to the basement and pull out a longer airlift tube. The one problem there is that when the power goes out, the pump reduces to minimum output, so I have to remember to test that mode, too - a backup pump moving no water in a blackout is a waste of money!
> ...Backup for me would simply aeration in culture tank independent of airlift in the event of power failure. Ideally it would be a pump that is also separate and powered separately from the pump driving the airlift.
Penn Plax does also have the B11 pump that does not turn on until it senses the AC power cut off, and then it runs with battery power - for only 48 hours vs. the AirPod's 150 hrs. But they're only $15, which is tremendously cheaper. That would be nice to have on an airstone for the fish and just use a plain AC pump for the airlift. The most powerful pump I have sitting around unused is their Silent Air X-4. It seems to make about the same air as the AirPod on low. I'll give that a test, too. Hopefully it makes enough air, because this would be a lot cheaper option.
Oh, why did I sell my Tetra Luft pump for next to nothing! Oh, because I needed to buy a car. >_> Well, at least someone up north is brewing beer with it...
Well, thanks for hitting me with all the questions and ideas. It's made me think a lot! I think this may be closer to my final setup:
NANFA Youtube Channel 
