Place the butter in a small bowl with the lime zest and juice. Mix it with a fork until well blended; set aside.
Take the fillets out of the fridge, pat them dry, and let them sit out until they are about room temperature—about 15 minutes. (This helps you get a more even cook on the fillet.)
In a small bowl, mix 1 tsp chili powder, 1/4 tsp garlic powder, 1/4 tsp cumin, 1/8 tsp onion powder, and 1/8 tsp cayenne; set aside.
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 evenly coat; set aside.
Heat the grill to medium-high heat (somewhere around 400-450º F). 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 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.)
When ready, remove the fillets from the grill and allow them to rest for a few minutes. Then slather them with the lime butter and enjoy!