Make the sauce. Mix the yogurt, salt, sugar, zest, onion, cilantro, and jalapeno in a small bowl and refrigerate for at least 30 minutes.
Prepare the fish. In a small bowl, mix cumin through onion powder. Brush fish with olive oil and sprinkle with spice mixture.
Grill the fish. Heat grill to about 400°F, place fish on the grill, and cook for 5 minutes. Carefully turn fish and cook for 4-6 minutes or until done. Remove fish from the grill and let it rest for 3-5 minutes. Use two forks to pull the fish apart.
Heat the tortillas. When you turn the fish, place tortillas on a grill to heat. If you have several burners on your grill, turn one to low so you can get the tortillas warm without making them too crunchy.
Assemble the tacos. Place 2-3 tortillas on each plate, divide fish evenly amongst the tortillas. Top with avocado, green onion, Queso Fresco, and lime-cilantro sauce. Serve immediately.