Bad fish shows itself through a handful of reliable visual cues: dull or sunken eyes, discolored flesh, slimy surfaces, and gills that have turned brown or green. These changes happen whether you’re inspecting a whole fish at the market, checking fillets in your fridge, or second-guessing leftovers. Knowing exactly what to look for can save you from a miserable bout of food poisoning.
Eyes and Gills on Whole Fish
The eyes are your fastest read on freshness. A fresh fish has clear, dark eyes with a metallic sheen. Once decomposition starts, the eyes turn grey, matte, and sunken. If you’re shopping at a fish counter or market where whole fish are on display, this single check tells you a lot.
Gills are equally revealing. Bright red gills mean the fish was slaughtered recently. As it ages, the color shifts to brown, then grey, then green. By the time gills look greenish, the fish is well past its prime. Lift the gill cover and look; fresh gills should also be moist, not dried out or sticky.
What Bad Fillets Look Like
Fresh fish fillets have a consistent, vibrant color. Salmon should be bright pink or slightly orange with clearly defined white lines running through the flesh. When it goes bad, it takes on a dull, greyish tone and those white lines fade. White-fleshed fish like cod or halibut will develop yellowish or brownish patches as they spoil.
Surface texture is the other giveaway. A thin, clear layer of moisture on fish is normal. Slime is not. If the fillet feels tacky, sticky, or coated in a milky or opaque film, bacteria have been at work. The flesh itself will also lose its structure. Fresh fish is firm and springs back when you press it with a finger. Spoiled fish feels soft, mushy, or flimsy, and a fingerprint stays indented. If a fillet of salmon breaks apart when you pick it up, it’s gone.
The Smell Test
Fresh fish smells like the ocean or like very little at all. The “fishy” smell people associate with seafood is actually a sign of decay. As bacteria break down fish tissue, they produce a compound called trimethylamine, which creates that sharp, pungent odor. The further along the spoilage, the stronger and more ammonia-like the smell becomes.
If you open a package and get hit with a sour or ammonia punch, that fish is done. This holds true for both raw and cooked fish. Trust your nose even before you look at the flesh.
How Cooked Fish Goes Bad
Cooked fish lasts three to four days in the refrigerator before it spoils. The signs are similar to raw fish but slightly harder to catch because cooking changes the texture and color. Look for a slimy or tacky surface developing on cooked fillets. The flesh may feel unusually soft or start to fall apart in a way it didn’t when first cooked.
Taste can also alert you. Spoiled cooked seafood often has a strong ammonia-like flavor, sometimes with a metallic or sour edge. If something tastes peppery or “off” when it shouldn’t, stop eating it.
Freezer Burn vs. Actual Spoilage
Freezer burn looks alarming but isn’t dangerous. It shows up as dry, whitish or greyish patches on the surface of frozen fish, caused by moisture escaping through gaps in the wrapping. The affected areas become tough and papery, but the fish is still safe to eat. You can trim off the freezer-burned sections and cook the rest normally.
Actual spoilage in frozen fish looks different. Watch for a condition called “belly burn,” where the ribs visibly separate from the flesh. Blood clots embedded in the meat, caused by poor processing before freezing, are another red flag. Fish with these issues wasn’t handled properly and shouldn’t be used. The general storage window for raw fish in the fridge is just one to three days. In the freezer, fattier fish like salmon and tuna stay good for two to three months, while leaner fish like cod and halibut can last six to eight months.
Worms, Cysts, and Spots
Finding a worm or a small cyst in fish flesh is surprisingly common and not automatically a sign of spoilage. Tapeworms, black spot (tiny dark specks under the skin), and yellow grubs are parasites that fish pick up in the wild. They look unappetizing but aren’t a health risk as long as the fish is cooked to an internal temperature of 145°F, which kills the parasites.
Red sores, open lesions, or unusual growths on the skin are a different story. These typically come from viral or bacterial infections in the fish itself. The damaged tissue should be cut away entirely. If a fish looks heavily diseased or was already dead when caught, skip it.
Bad Fish That Looks Fine
One of the trickiest dangers with fish is that some contamination is invisible. Histamine toxicity, commonly called scombroid poisoning, is the most common chemical cause of seafood-related illness in the United States. It happens when fish that should be kept cold sit at temperatures above 40°F for too long. Bacteria convert a natural amino acid in the flesh into histamine, which builds up to levels that trigger an allergic-type reaction when eaten.
The fish can look and smell completely fresh. The only clue during a meal is sometimes a peppery or unusual burning taste. After cooking, the skin of contaminated fish may look honeycombed. The species most prone to this are dark-fleshed fish: tuna, mackerel, bonito, mahi-mahi, bluefish, sardines, and yellowtail.
Symptoms typically appear within two hours of eating and include facial flushing, headache, diarrhea, and a rash that looks like hives. Most cases resolve within 12 to 48 hours, but people with asthma or heart conditions can have severe reactions including breathing difficulty. Because the fish appears normal, the best prevention is keeping it cold from the moment of purchase. If it sat out at a buffet, in a warm car, or on a counter for more than a couple of hours, the risk goes up regardless of how it looks.
Quick Reference by Sign
- Grey, sunken eyes: early decomposition in whole fish
- Brown or green gills: no longer fresh
- Dull, greyish flesh: spoilage in fillets, especially salmon
- Slimy or tacky surface: bacterial growth, raw or cooked
- Mushy texture that doesn’t spring back: breakdown of flesh
- Ammonia or strong fishy smell: advanced decay
- Dry, white patches in freezer fish: freezer burn (safe but lower quality)
- Ribs separating from flesh in frozen fish: severe deterioration
- Peppery taste or honeycombed skin after cooking: possible histamine contamination