Heat a grill or grill pan to medium-high. Clean and lightly oil hot grill. In a medium bowl, whisk together olive oil, garlic, and lime juice. Add fish and turn once to coat; let sit 5 minutes.
Remove fish from marinade, letting excess drip off. Season with salt and pepper and grill until flesh is opaque throughout, 12 minutes, flipping once. With a fork, break fish into large pieces and serve with tortillas and corn relish.
In a heavy skillet, toast tortillas over medium until browned in spots. Or, wrap a stack of them in parchment-lined foil and warm in a 400-degree oven for 5 minutes.