Preheat the air fryer. Set it to 400°F (200°C). Give the basket a light spray with oil or brush it with olive oil to prevent sticking.
Make the garlic butter. In a small bowl, whisk together melted butter, minced garlic, lemon zest, lemon juice, parsley, smoked paprika, onion powder, salt, pepper, and red pepper flakes if using.
Prep the swai. Pat the fillets very dry with paper towels. Dry fish browns better and won’t steam in the air fryer.
Season generously. Brush both sides of each fillet with the garlic butter mixture.
Reserve 1 to 2 teaspoons for a quick finish after cooking.
Arrange in the basket. Place fillets in a single layer, not touching. Work in batches if needed so air can circulate freely.
Air fry. Cook for 7 to 10 minutes, depending on thickness. Thin fillets often finish around 7 to 8 minutes; thicker pieces may need up to 10 minutes.
Check doneness. The fish should flake easily with a fork and reach an internal temperature of 145°F (63°C).
The surface should look opaque and lightly browned.
Finish with butter. Brush the reserved garlic butter over the hot fillets for a glossy, flavorful finish.
Serve. Add a squeeze of fresh lemon and garnish with extra parsley. Pair with steamed vegetables, rice, or a crisp salad.