Season salmon fillets with lemon pepper, garlic powder and salt. In a small bowl, stir together soy sauce, brown sugar, water and vegetable oil until sugar is dissolved. Place fish in a large resealable plastic bag with the soy sauce mixture, seal and turn to coat. 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.