How Long to Boil Fish
Fish fillets need about 8 to 10 minutes to fully cook in hot water, while a whole fish needs 15 to 20 minutes. In either case, you'll want to keep the water at a low boil, closer to a poach, rather than a full, rolling boil. Use just enough liquid to submerge the fish, and cover the pan tightly as the fish cooks. Fish is fully cooked when the color changes from translucent to white and flakes easily with a fork. You can also take its temperature: You'll be looking for your thermometer to read 145 degrees.