Heat 2 tablespoons of the oil in a large nonstick skillet over medium-high heat. Season the fish with the coriander, ½ teaspoon salt, and ¼ teaspoon pepper.
Cook until opaque throughout, 2 to 3 minutes per side. Transfer to a plate and break into pieces.
Combine the grapefruit, avocado, the remaining 1 tablespoon of oil, ½ teaspoon salt, and ¼ teaspoon pepper in a medium bowl.
Top the tortillas with the fish and grapefruit salsa, dividing evenly. Serve sprinkled with the chives.