For a 6-ounce fillet (about the size of your palm):
White fish (cod, tilapia, haddock): 2 to 3 minutes on high. Salmon: 3 to 4 minutes on high. Dense fish (swordfish, tuna steak): 4 to 5 minutes on high, then rest for 1 minute. Adjust for thickness—if it’s over an inch, add 30 seconds.
Halfway through cooking, rotate the plate 180 degrees if your microwave doesn’t have a turntable. Even heat distribution is your best friend here.