Because it's easy to overcook and dry it out, cooking fish the first time is a delicate enough endeavor, but reheating leftovers is even trickier. Leftover fish dries out a little in the fridge or freezer, and it's likely to dry out even more during reheating. Don't overlook options such as adding cold fish to a salad or reheating it and using it flaked in a pasta dish to mask some loss of quality. Otherwise, "low and slow" is your best bet.
The Best Way to Reheat Fish
A gentle approach – a longer cooking time at a lower cooking temperature – is the best way to reheat fish. Do it in the oven (or toaster oven if you don't want to bother with the larger appliance just for some leftover fish) at 275 degrees Fahrenheit for about 15 minutes. Fish should be reheated to an internal temperature of 125F to 130F. You could also do it on the grill over low indirect heat if you're already using it and it's no extra trouble.
The other key to reheating fish is to wrap it loosely in aluminum foil while it's heating up. This holds in steam during cooking, helping the fish retain moisture and not dry out on the exterior. Boost this effect by adding a few teaspoons of water or broth into the foil wrap.
Generally speaking, thicker cuts hold up better than thinner ones during reheating. So, for example, reheating salmon or swordfish steaks results in a more moist, flavorful finished product than reheating tilapia, trout or flounder fillets. However, reheated fatty fish like salmon and mackerel can give off a slightly pungent odor. That's normal; it doesn't mean the fish is bad.
Reheating Frozen Fish
Warming up frozen cooked fish is even more precarious that reheating refrigerated leftover fish. Thaw it first but not in the microwave, or it'll end up dried-out, chewy and unappetizing. In fact, while we're on the subject, don't reheat any leftover fish in the microwave for the same reason.
The optimal safe thawing method is to put the frozen cooked fish in the refrigerator for about 24 hours; this yields the least moisture loss during thawing. A second option is to submerge it in cold water in a leak-proof package for 30 minutes to a few hours, depending on the quantity and type of fish. If it takes longer than half an hour, pour out the water and put in new cold water every 30 minutes.
Once it's thawed, reheat fish in the oven or toaster oven as you would refrigerated cooked fish. Wrap it loosely in aluminum foil, add a few teaspoons of water or broth, and heat it in the oven or toaster oven at 275 degrees F for about 15 minutes, until it reaches an internal temperature of 125F to 130F.
Storing Leftover Fish
Properly storing cooked fish helps it reheat with the best possible results. Keep it in an airtight container in the refrigerator for three to four days, tops. Freezing it will take a toll on its quality, but if you want to keep it longer than a few days, this is an option.
To freeze cooked fish, store it in an airtight container or freezer bag, forcing out as much air as you can while sealing it. Air exposure leads to freezer burn. Frozen food technically stays safe to eat indefinitely, but quality degrades more and more over time. Date the package and try to eat the leftover fish within four to six months.
If your leftover fish has a bad smell or if it appears discolored or slimy, it's gone bad and should be discarded. Toss it out if it gets freezer burned, too.