In a small saucepan, bring 1 1/2 cups salted water to a boil; remove from the heat, stir in the couscous, cover and let stand for 5 minutes. Using a fork, stir in 1/2 cup parsley, salt and pepper. Keep warm.
Meanwhile, in a large nonstick skillet, heat 2 tablespoons olive oil over medium-high heat. Season the fish with salt and pepper, add to the pan and cook, turning once, until golden-brown and just cooked through, 5 to 7 minutes. Transfer to a serving platter, squeeze a wedge of lemon over each fillet and tent with foil to keep warm.
Using the same skillet, heat the remaining 1 tablespoon olive oil. Add the bell peppers and onion and season with salt and pepper; cook until softened and golden-brown, 6 to 8 minutes. Spoon the vegetables over the tilapia and sprinkle with the remaining 2 tablespoons parsley. Serve with the couscous and remaining lemon wedges.