How to Preserve the Slime Coat on Fish

Slime on fish skin helps them move through the water and protects the skin
Slime on fish skin helps them move through the water and protects the skin.

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If you’ve ever touched a fish, you know that it feels slimy. If you knew what that shiny, slimy stuff does, and can get past the ick factor, you may wish you had a slime coat too!

Fish slime coat is a marvelous protective barrier that helps maintain good health in fish. Unfortunately, we often unwittingly do things that damage that wonderful barrier. Here’s why it is important to take steps to protect the slime coat of your fish.

What Is Slime Coat?

The slime coat in fish is composed of a glycoprotein (protein with attached carbohydrate) that serves as the frontline barrier to virtually everything from large physical objects to tiny bacteria. This barrier also works to keep essential fluids and electrolytes in the fish, and helps the fish glide through the water by reducing surface resistance.

Much like humans have various layers of skin, fish have multiple skin layers as well. In their case, they have skin (dermis) that produces a layer of scales. The scales, in turn, are covered by a thin layer of epidermis. Goblet cells in the epidermis produce slime. Any break in the slime coat is similar to an abrasion on our outermost layer of skin. Losing a large portion of their slime coat would be like damaging a large portion of our skin.

How Slime Coat is Damaged

Anytime something brushes against the slime coat; it is disturbed. Handling, hooking or even netting a fish causes a significant disturbance of the slime coat. Biting or nipping by other fish also can cause slime coat damage. However, physical assaults are not the only thing that can damage the slime coating.

Any stress can, and will, impact the protective coating of the fish. Low oxygen levels, temperature changes, and elevated toxins (e.g., chlorine, ammonia) in the water will reduce the protective slime coat. Changes in water composition, such as with salinity, pH or hardness are other possible contributing factors to slime coat damage. Parasites on the skin cause irritation that increases the production of slime by the fish, changing its appearance to have a white or bluish tint to the skin.

Impact of Slime Coat Loss

As previously explained, the slime coat covers the entire surface of the fish and is much like the outer layer of human skin. If it is damaged, it is similar to a burn or scrape in a human. However, fish can’t put bandages on their slime coat to protect the skin. That leaves the fish wide open to disease and parasites.

Many fish diseases are caused by bacteria that are always present in the water. Normally these organisms can’t get through the slime into the fish, but when the slime coat is broken or stripped off, the bacteria can overwhelm the fish like enemy warriors pouring through a broken gate in a castle. Soon the fish is overrun with bacteria that it cannot fight off. Likewise, many parasites are only able to get into the fish if the slime coat is first damaged.

Lastly, the slime coat maintains electrolyte balance and keeps proper fluid balance. A fish that has lost the slime coat has side effects similar to a human who has been badly burned, losing essential minerals into the surrounding water. Freshwater fish will absorb water through the damaged skin and can get over hydrated and bloated, while saltwater fish will lose body fluids into the surrounding water and become dehydrated.

Steps to Take

Although it may not be possible to avoid it completely, the best course of action is to not damage fish slime coat in the first place. Avoid handling fish whenever possible, which includes netting them. If you can scoop a fish up using a glass, instead of a net, you will cause far less damage to the slime coat. If you have to use a net, once you have caught the fish in the net, keep the net in the water and place a cup or bowl under the net with water in it, then lift the cup of water containing the fish in the net so that the fish is never out of the water while it is being transferred into another aquarium or into a plastic bag for transport. This avoids the fish flopping in the net while held in the air, which is more likely to damage the slime coat and skin. In general, don’t touch your fish. If you must, wet your hands first, or wear smooth latex or vinyl gloves that have been wetted, or use a soft damp cloth to minimize the trauma.

Maintain good water quality at all times. Poor water quality is one of the leading causes of fish stress, which in turn, damages the slime coating. Perform regular water changes, keep the tank clean, test the water regularly, and take steps to mitigate rising toxins such as ammonia. Do not allow the water to rapidly change temperature, as this is also a major stress factor. Whenever new fish are introduced to an aquarium, turn the lights off for a few hours to calm the newcomers as well as the old-timers in the tank.

Any time a fish is under stress, there is a risk the slime covering will be impacted. Fortunately, there are aquarium products available at your fish store that promote healthy slime coat and also provide soothing relief to damaged coatings. These products contain polyvinylpyrrolidone or aloe vera, which attach to the fish's skin to improve the slime. Use of these products is good insurance against stress and disease, especially when transporting fish.