When it comes to fruit, it's easy to see if there's something so wrong with it that it shouldn't be eaten - it could be discolored or just clearly rotten, but it's a lot harder to tell if there is a problem with food that is packaged, like fish. However, the FDA has issued a warning to anyone who eats fish, alerting them to a big red flag they need to be on the lookout for.
According to the Food and Drug Administration, if you see ice crystals on the packaging of frozen fish, do not buy it, and if you've already purchased it, whatever you do, do not eat it. It may seem strange to avoid ice crystals on frozen food but the agency explained that the crystals are evidence that the fish isn't fresh because it has been in storage for a while, or that it was thawed and refrozen, allowing for dangerous bacteria to grow on the fish due top the temperature changes.
The FDA also recommends not buying or using any frozen seafood with packaging that has been torn, opened or has edges that are crushed. Additionally, if you touch the flesh of frozen fish and it doesn't feel solid, avoid it.
Meanwhile, if you buy your fish fresh, the store should have it on a bed of ice, ideally enclosed in a case. Also, even if you see nothing wrong with the fish, the nose knows if something is up too. If your fish has an overwhelming fishy, sour or ammonia-like odor, you should't eat it.