Put the chopped fish in a shallow bowl, pour the melted butter over top, and toss around to coat. Sprinkle in the seasoning and toss again until well-coated.
Place the seasoned fish pieces into the hot skillet and cook until the fish starts to become flaky and is no longer transparent on the bottom, 3 to 4 minutes. Carefully flip over the pieces and cook until opaque and flaky all the way through, another couple minutes.
Serve immediately with the tortillas, mango, and other toppings.