Take the fillets out of the fridge, pat them dry, and bring them to room temp. (This helps you get a more even cook on the fillet.)
In a small bowl, mix the spices together from the chili powder to the salt and pepper. Taste, and adjust seasoning if needed.
Place the fillets in a bowl, drizzle the olive oil over each fillet, and sprinkle the seasoning blend over the fillets. With your bare hands, gently toss the fillets in the oil and seasoning to coat; set aside.
Heat the grill to medium-high heat (somewhere around 400-450º F). Once the grates are heated, lightly dip a bunched up paper towel in oil. Holding the paper towel with tongs, drag it across the hot grates to create a slick surface. Continue to wipe the grates with the oiled paper towels, re-dipping the towels in oil between applications, until the grates are black and glossy. Depending on your grate, this could take between 5-10 swipes. Building a coating on the grates will prevent your fish fillets from sticking.
Place the fillets on the grill (skin-side down if your fillets have skin) diagonal to the grate slats. Reduce the heat to medium, and cover the grill. Let the fish cook without disturbing it or moving it until the skin is golden brown and crisp—about 2 to 4 minutes.
Try gently sliding a spatula beneath the fish. If it doesn’t easily release itself from off the grill, don't force it. Let it continue to cook, checking it every 30 seconds, until it releases. At that point, carefully flip the fillet. (If you don't have a proper wide fish spatula, you may want to use two spatulas for flipping.)
Cover the grill and let the fillets cook until the centers of the fillets are opaque and register 140º F on an instant-read thermometer. (This can take between 3 to 7 minutes depending on the thickness of your fillet. The ideal internal temperature for fish is 145º F, but protein will continue to cook once removed from the heat, so I like to take it off a little early to ensure I have flaky and tender perfection.
When ready, remove the fillets from the grill and allow them to rest for a few minutes.
Slather lime butter over the fillets and enjoy.