Why are fish covered in mucus? Well there plenty of reason for this, mucus plays an important role in the survival of some fish. It’s secreted from cells under the scales and skin; these cells sometimes are located in different parts of the fish, indicating its species. The cells will produce glycoprotein which mixed with water will result into a slippery mucus.
Fish mucus is important because it helps regulate body functions, including protection against external parasites. Some parasites won’t be able to attach themselves to slippery scales; others will choke on mucus excess. Also, some studies showed that mucus will help the fish swim around faster, slowing water movement around the fish.
Some fish mucus will contain toxins. If any creature bites the fish or eats it, it will suffer some consequences and next time will think twice before taking action. The parrotfish for example will produce a bubble made out of mucus and will wrap its whole body in it, protecting itself from possible attacks.
Fish breathe through gills right? They also breathe throw scales and skin; the mucus will allow the chemical molecules to circulate inside the fish and vice versa.
Some aquarium fish, like the Tetras, Rasbora and Labrids have some special cells and when these cells are broken (will happen when damage occurs to the scales and skin) a hormone of fear will be released into the tank, most of the fish will start to freak out with no apparent reason, sometimes bouncing into the aquarium glass or decoration without any visible reason. These substances are not specific to one species when it comes to the effect; it appears to be an alarm for all fish in the aquarium. Only a massive change of water (at least half) will remove these chemicals and allow the fish to calm down.
Some fish species will secret mucus with which they’ll feed their young. This mucus is often full of protein and fats, things the fry need to grow and develop. The famous Discus is the best example.
Always check aquarium fish before you buy them, be sure they haves no wounds, sores or discharges of mucus. These symptoms may indicate the fish is sick or dying. It is difficult for a fish to recover after suffering damage to their skin or to its mucus cells. In some cases parasites may enter their wounds. However it’s important to keep them in quarantine, like for all fish before you introduce them in the community tank. Any new comer must be held at least 5 days in quarantine, check for any signs of disease or any abnormal things, this way you will prevent mass illness or death among your healthy fish.