Help! I've got a sickly pepper plant.

We're (my fiancee and I) growing a small urban planter-garden in the sunlit space below the stairs to the apartment above ours. I'm not particularly green, but regular water, occasional fertilizer, and Los Angeles weather seem to be more than ample for growing.

We picked up three pepper plants: habanero, cayenne, and banana. The cayenne is going balls out, and the habanero's a little puny due to the holy-crap-I-want-to-kick-this-kid's-face-in 6 year old living upstairs ripping the top leaves off (at least, that's the going hypothesis; I've already caught him terrorizing the tomatoes).

The banana pepper was looking small but healthy until yesterday afternoon. Now, it's "wilty" like no one's business. It receives direct sunlight for hours a day and is watered with its buddy plants. I've been tossing it tomato food (almost through the bag) at even intervals. Any idea what would be destroying the plant? It's just so sudden.