Well, I have put an end to my rooftop hydroponics experiment. While the air pump was bubbling in the hydroponic setup on the roof, I had kept one lettuce back, mostly because there was no more space for it in the tray on the roof. This one, I left on my windowsill and hand watered just about every day, just enough to leave a little pool of water in the holding tray. Every day, the lettuce either used up the water or it evaporated. The result is the much larger lettuce you see on the left side here. It had been planted at the same time as all of the others but is clearly much larger! I think that means the rooftop hydroponic setup was a failure. I think it is probable that there was simply too much water and the plants were drowning. They need more air around their roots to breathe. The only other thing I can think of is maybe the roof is too sunny for lettuce to grow nice big leaves? I have read in some places that some plants grow smaller leaves when they have lots of sun, but I'm guessing it was mostly the drowning.