Season salmon fillets with lemon pepper, garlic powder, and salt to taste. In a small bowl, stir together 1/3 cup soy sauce, 1/3 cup brown sugar, 1/3 cup water, and 1/4 cup vegetable oil until sugar is dissolved. Place fish in a large resealable plastic bag with the mixture. Refrigerate for at least 2 hours. Preheat grill for medium heat. Lightly oil grill grate. Place salmon on the preheated grill, and discard marinade. Cook salmon for 6 to 8 minutes per side, or until the fish flakes easily with a fork.