Heat the olive oil in a large pan over medium-high heat.
In a small bowl, mix together the chili powder, cumin, salt, and pepper.
Season the perch fillets with the spice mixture, then place them in the pan. Cook the perch for 3-4 minutes on each side, or until it flakes easily with a fork. Remove the perch from the pan and set it aside.
In a medium bowl, mix together the mango, bell pepper, cilantro, red onion, and lime juice.
To assemble the tacos, place a piece of the perch on each tortilla and top it with a spoonful of the mango salsa. Add a dollop of sour cream, if desired. Serve the tacos hot.