Slime in Fish refers to the protective layer on the fish's body. The outer skin of fish produces glyco-protein. This glycoprotein combines with water to produce the slime on the fish's skin. Slime protects fish from predators by making them slippery.
Slime also protects the fish from injuries and from infections. Slime helps heal injuries on the fish's body heal. Slime also reduces drag and helps the fish overcome water resistance while swimming.
Slime can be damaged by the use of soap. Never clean the aquarium or any of the equipments of the aquarium such as filters, pumps, etc. with soap.
Slime also protects fish from parasites. In fish species, such as the Discus, a slime substance is secreted by the female to feed her babies.
The Slime coat in a fish can be damaged by poor water quality. This makes the fish vulnerable to infections and parasites.
Handling fish incorrectly while catching can also damage the slime coat. Unnecessary handling should be avoided. Fish should be caughtly gently with a large net.
Parasites such as anchorworms and fish lice can also affect the slime coat.
There are medication which provide a synthetic slime coat on the fish particularly when fish are handled or netted. You can add these medication when fish are introduced into a new tank or after they have been handled.