In a medium nonstick skillet, bring 1/4 cup water to a boil. Add tomatoes and red-pepper flakes; top with tilapia, and season with salt and pepper. Cover, and cook 3 minutes. Add arugula. Cover, and cook until tilapia flakes easily with a fork, about 2 minutes. With a slotted spoon, transfer tilapia and vegetables to a plate (reserve skillet).
Make sauce: Off heat, add butter, lemon juice, and capers to skillet; swirl until butter has melted. Season sauce with salt and pepper. Serve tilapia over arugula and tomatoes; drizzle with sauce.