Preheat your air fryer to 380° F for 5 minutes. Preheating helps ensure even heating and better results. Place the leftover fried fish in a single layer in the air fryer basket. Do not overcrowd the basket; leave some space between the pieces for air circulation.
Cook for about 3-4 minutes total until warmed on inside and crispy on outside. The exact time can vary depending on the thickness of each piece and the air fryer model, so keep an eye on it.
After about half of the reheating time has passed, flip the pieces over and spray lightly with olive oil spray to ensure even heating on both sides. After 4 minutes or so total if it isn't crispy enough, flip over once more, spray lightly with olive oil spray on the other side and air fry 1 more minute or until crispy to your liking. Do not overheat or it will dry out in the middle.
Once heated through and crispy to your liking, remove it from the air fryer, and serve immediately.