There is a reason that fish are so slimy, and it's very important for the survival of fish. Slime is secreted from cells in the very outside layer of the skin. In some cases, the placement of these slime-producing cells determines what species of fish it is. The cells produce what is called a glyco-protein, which is then mixed with the water making the slimy mucus.

Slime is important for fish to regulate many necessary body functions, including protection against parasites. Some parasites can't attach to the scales because they are too slippery; others suffocate in the slime. Slime also acts as a band-aid, covering wounds and scrapes. Some studies indicate that slime may aid in reducing the turbulence along the body, making the fish faster in the water.

Some fish also take advantage of their coating and lace it with a toxin. If some creature takes a bite, it may suffer the consequences and think twice the next time. Some fish form cocoons out of their slime. Parrotfish, for example, produce a mucus "tent" that completely envelopes them, which may protect them from attack. Slime can also regulate, on a molecular level, the exchanges that are made between the fish's body and the surrounding seawater. Fish "breathe" with their gills, but they also breathe through their skin. The slime allows the necessary chemical molecules to pass between the inside and outside of the fish.

In some aquarium fish such as tetras, barbs, sharks, rasboras and loaches among others, the skin contains a number of cells that have no opening to the outside, called "blind cells." When these cells are broken, which occurs by a rupture in the skin, they release a fright hormone that is transmitted through the water and sends an alarm of sorts out to other fish. Suddenly these fish will dart about, sometimes ramming into the sides of the tank or decorations for no apparent reason. These substances aren't species specific, but seem to be an alarm for all. Only a water exchange (at least 3/4 partial exchange) will solve this problem once these fright substances are in the water. They must be chemically removed and won't be filtered out through your system.

Several fish secrete body slime with which to feed their young. Some of it can be high in proteins and fats which is what a young growing creature requires.

Body slime diseases and ailments should be taken seriously. Always inspect any fish that you are considering for purchase and make sure that you do not see any lacerations, swellings, or slime streaming from its body. This could indicate a sick or dying fish. Deteriorating slime and skin is very difficult to recover from.

In some cases, parasites may have gotten to a wound. In any case, it is important to quarantine any fish you are considering to add to your community aquarium. Keep the newcomer on his own for about five days and watch him. Observe his body and watch for any indications of disease or abnormal conditions-such as streaming slime. Such diseases will quickly spread to your other fish.