1. Cook couscous according to package directions, with salt.
2. Combine the cumin, chili powder, coriander, and a pinch of pepper. Evenly rub the spice mixture on all sides of the fish and set aside.
3. Combine mango, cilantro, and lime juice together in a medium-sized bowl and set aside.
4. Heat the butter in a large saute pan over medium heat. Add the seasoned fish to the pan and cook on both sides until it's translucent (3-4 minutes per side).
5. Once the couscous is done, add it to the bowl with the mango, cilantro, and lime. If desired, stir in any juices from the pan and top couscous with fish to serve.