Thai Shrimp Halibut Curry

1 tablespoon vegetable oil

1 cup chopped shallot s

1 large red bell pepper , cut into 1/2-to 3/4-inch dice

1 1/2 tablespoons minced peeled fresh ginger

2 1/2 teaspoons Thai red curry paste (such as Thai Kitchen brand)

1 13 1/2- to 14-ounce can unsweetened coconut milk

1 tablespoon fish sauce (such as nam pla or nuoc nam)

16 to 18 ounces halibut fillets, cut into 1 1/2-inch chunks

8 peeled deveined uncooked large shrimp (8 to 10 ounces)

1/3 cup chopped fresh cilantro

1/3 cup chopped fresh basil