There's easier ways of doing hydroponics where you get a single reservoir to grow in and put in an airstone, it doesn't have to cycle. Look up Kratky Hydroponics if you're interested. I might do it in the future with kitty litter buckets for tomato plants as they would be too large for this ebb and flow type system I built.

As for what happened on your balcony, I would think either you were facing north or forgot to water your lettuce. Balconies don't get much moisture especially if they are on higher floors.