Place leftover cooked cod in a baking dish and pour a little bit of water into the bottom of it. Cover with aluminum foil and bake in a low oven (around 300°F) for around 10-15 minutes, or until the cod is sufficiently warmed throughout. While reheating it on lower power levels in the microwave can work in a pinch, you risk the cod drying out. Not to mention that reheating fish in the microwave is notoriously fragrant.