Fish have less developed immune systems than humans, but they live in a world at least as germ-ridden as ours. Bony and jawless fish generate a coating of mucus to protect them from germs in their environment. Sometimes, one of these specimens will shed this slime coat. It's a sign of poor health.
What Is a Slime Coat?
A slime coat is a covering of mucus. Most bony and jawless fish have this coating. The mucus protects their skin from germs, viruses and other parasites. It acts like a second skin, a barrier against these troublesome organisms. Additionally, the mucus may contain various enzymes that make it harder for parasites to survive on fishes' skin. It's not a foolproof defense mechanisms against parasites. On a healthy fish, the slime coat usually appears clear, and you will probably not even be able to see it.
Shedding Slime Coats
Sometimes, fish shed their slime coat when it gets infected or irritated. When this happens, the slime coat tends to get discolored or cloudy. Fish will shed their coats to try to "shrug off" whatever's irritating their skin. This will typically look like stringy, clear or cloudy strands trailing off the fish. Since shedding slime is a symptom, not a disease, it often accompanies other symptoms such as skin ulcers or injuries.
A number of things can cause a fish to shed his slime coat. They include infections, including viral, bacterial, fungal and protozoan. Larger parasites like skin flukes may irritate the skin and cause the fish to produce excess skin mucus. Bad water chemistry is another cause for a fish to shed his slime mucus. Water that contains chlorine or dissolved minerals and water at the wrong pH for the species are causes. Physical damage to the skin, such as abrasions from transport or fighting can trigger shedding of the slime coat.
If you see your fish shedding slime, it's most likely a sign of a problem. But you can prevent it a number of ways. Keep up on your regular water changes. This keeps the water chemistry proper. If your local tap water has dissolved phosphates or metals in it, use reverse-osmosis filters to produce pure water. If you see signs of infection, treat them immediately with aquarium medications. You can pick these up at most pet shops. You can also help replace slime coats with a product called stress coat. This is a synthetic substance you add to the water. It coats fish and forms an artificial slime coat for fish are too stressed to produce their own.