Mix together the chili powder, paprika, cumin, garlic powder, onion powder, salt, black pepper, oregano, and cayenne if using. Store in an air-tight container in a cool, dry place. To turn this into a marinade, add fresh lime juice and olive oil.
Use about two tablespoons of seasoning for every pound of fish (cod, tilapia, halibut, Mahi Mahi, salmon, etc) for tacos. Rub right onto the fish and cook in a skillet, in the oven, or on the grill until flaky and opaque. For even more flavor, also drizzle fresh lime juice over the fish before adding the spices.