Fish is a great source of protein (not to mention healthy fats, vitamin D, and potassium), but freshness—and correct storage—is key. (You won't get much nutritional benefit from fish that makes you sick.)
MORE FROM HEALTH.COM:
These five easy-to-remember rules will help you pick out the freshest fish and make sure it stays that way till it hits your plate.
1. Follow your nose
Buy fish from a clean store that doesn't smell bad, where the fish is displayed on ice, not in packages. This might be the counter at a supermarket or a fishmonger. Once you find that place, make friends.
2. Be flexible
For every fish you can think of, there are others much like it; they're interchangeable. So instead of focusing on individual species, think of fish as belonging to one of three main groups (thick fillets, thin fillets, and steaks), then choose what looks best in each category.
3. Be picky
Buy what smells briny and sweet—like the sea—not stinky or acrid like chemicals. Also, avoid anything mushy, dried out, or "gaping"—where the flesh is separating.
If you're not sure, ask your new friend behind the counter to let you have a whiff and a close look.
4. Choose safe and sustainable seafood
Many species of seafood have been overfished or polluted to near extinction, and farmed fish isn't always the best alternative. Several reputable organizations can help you decide what fish and shellfish to avoid, using lists that fluctuate along with the fish populations themselves.
Check out Monterey Bay Aquarium's Seafood Watch for one.
5. Keep seafood cold
To help maintain good quality, hurry home (or have the store pack the fish in ice). Cook thawed seafood within a day. Frozen seafood degrades with time, so eat it within a couple of months.