Our fish are slimy. If you’ve ever bumped one of your aquatic charges with your hand, or picked one up, you’ve noticed that layer of slime coating. Many additives that are intended to reduce stress in captive fish also claim to enhance the slime coat. Believe it or not, this coating of slime is an aquarist’s best friend. Not only does it help ward off potential parasites, it allows damaged tissue or fins to regrow. Yet many captive fish have insufficient slime coats. Why is that? What factors aid in generating an adequate mucus coating, and what can we as aquarists do to enhance this beneficial, and crucial part of our fishes’ biology? 

The slime coating:

If you’re a student of evolutionary biology, then it’s important to accept that somewhere along the line we evolved from fish. Life began in the sea, and over the millennia evolved to occupy land. When this took place our slime glands evolved into sweat glands, since we no longer had a direct need for mucous producing glands within our skin. Yet mucous didn’t go away altogether. If you’ve ever had a cold or respiratory infection, mucous has filled your sinuses and nasal passages, rolling down your throat and clogging up your chest. This mucous works much like a fish’s slime coat. It captures bacteria, fungus, viruses and parasites and pushes them out via sneezing or coughing, or a runny nose. For humans, the downside to this mucous defense is that it can clog up within us, giving bacteria an optimal growing condition and leading to infection.

One of slime’s obvious benefits for fish is that it reduces drag by coating an irregular surface such as scales. This allows fish to slip easily, and quickly through their environment. Slime also affords fish protection from fungi, bacteria and ectoparasites. The slime coat traps the parasites and naturally sloughs them off as water moves around the fish. Within fish slime are a host of immunities, so if the parasite doesn’t slough off within the slime, there may be an antibody that kills it. While this sounds like the perfect defense mechanism, parasites evolved right beside the fish, and many have an innate ability to penetrate the slime coat and burrow into fish tissue.

A slime coating also soothes a fish’s open wounds, with highly effective medicinal properties. It’s so effective in fact that human researchers are trying to isolate the active components of fish slime for use in human medicine. It’s believed that a fish’s slime coat could treat infections and speed up the time it takes wounds to heal. Some researchers are recommending hagfish slime be used for clothing, or as a replacement for technical clothing like lycra. For fish, slime coat creation is a function of the immune system, just like mucous creation is a function of ours. If fish are exposed to irritants such as copper, then automatically they produce a thicker slime coat. The slime coat also aids fish in balancing electrolytes, by creating a two-way, living osmoregulatory filter. Since fish exchanges gases at their skin’s surface, the slime coat actually enhances this gas exchange. If a fish’s environment rapidly changes salinity, they will generate more slime to aid in balancing internal bodily salt content with the outside environment.

Some species’ slime coats contain toxins that immobilize, or even kill potential predators. There are species with toxic slime capable of stopping a shark bite mid-chomp. Other species use enhanced slime coats to build nests, and African lungfish secrete glue-like slime that allows them to form a cocoon within the soil, keeping the fish moist till the rainy season returns.

For aquarists, this slime coat presents our best defense against unwanted external parasites, but often captive fish don’t have a slime coat nearly as strong as their wild counterparts.

Maintaining healthy slime:

A host of environmental conditions can deteriorate a fish’s natural slime coat. Netting a fish can break open the slime coat, giving parasites and bacteria a direct port to the fish’s tissue. Poor water quality can decrease fish’s natural immune system function, resulting in an incomplete slime coat. Water changes that are of the wrong temperature or salinity can at first increase slime coat production, but in the long term decrease immune response. Any of these factors reduces a fish’s ability to generate slime, and increases the opportunity for bacteria and parasites to infect the tissue.

Believe it or not, various aquarium additives can damage the slime coat. If you’re using ozone, not properly filtering the reactor (of skimmer’s) affluent can allow some of the gas to seep into the water, causing slime coat damage, along with a host of other biological unrest. Medications can both increase slime coat production and damage it. When the slime coating is damaged, fish bleed electrolytes into the water. As you can imagine, this throws osmoregulation on its head and can lead to all sorts of health maladies for the fish.

Yet diet is the ultimate tool in maintaining a healthy slime coat, or increasing a fish’s overall immune system. Many aquarium fish aren’t fed the appropriate amount of food, nor the right kinds of food. The captive environment is very unnatural for fish, in the sense that fish are housed closely together often with species they wouldn’t encounter in nature. To make matters worse, pumps, skimmers and other filtration equipment makes noise and interfere with fish’s ability to navigate. Some aquarists have suggested that the aquarium environment also overloads fish’s lateral line, leading to head and lateral line erosion (HLLE).

The best way to ensure fish are getting enough food, is to feed a small amount of food throughout the day. Once an hour, or once every two-hours, offer fish a small pinch of food, ensuring that their stomach is always filled with nutrients. Fish in the wild will eat and eat, even if their mouths and stomachs are full. Many of us have noticed our surgeonfish filled up, ready to burst from gorging on freeze dried seaweed.

Feeding the right foods is equally important. You wouldn’t feed a grouper freeze dried seaweed, and you wouldn’t feed a tang live mollies. It’s for this reason that frozen food blends are overtaking pelleted foods in popularity. We’ve learned that over processed foods, and filler items such as fish meal, don’t really meet the nutritional requirements of marine fish. There is argument and speculation about how fish digest processed foods, and how well they meet species’ nutritional needs.

Another area that could use more research is how probiotics effect fish digestion, and if they aid in nutrients getting absorbed, and then used for slime coat generation. Like any immune system function, slime coat generation takes energy, and that energy requires calories as a catalyst.

The general school of thought is that probiotics enhance beneficial gut flora, which helps the body digest. Enhanced digestion ensures that food’s nutritional content is maximized. Could probiotics in fish food indirectly aid in the generation of a strong, viable slime coat?

When we acquire fish:

Everything about capturing, shipping and housing marine fish damages their natural slime coat. Fish are captured from the ocean, and often during that time they flee and dart around, hitting rocks and sand. This causes breaks in the slime coat, as does the physical act of netting. Fish are then placed in close quarters with little to no food, so their bodies are unable to re-create a viable slime coat. During shipping, water conditions change quickly, causing all sorts of immune reactions within fish and potentially wreaking havoc on fish’s slime coat. The bottom line, when we receive fish, it’s more than likely their natural slime coat is in disrepair. Bringing that slime coat back up to normal standards is another strong argument for quarantine. There are several compounds known to help re-generate a fish’s slime coat, but their effect on corals and invertebrates is questionable, making it wise to only use them in a quarantine environment.

Often aquarists spend a lot of time medicating new arrivals, which can be a valuable practice. However, a natural slime coat is the best defense (and offense) against parasites. Also, a viable slime coat offers long-term protection against infection. When quarantining fish, these compounds can aid in re-generating a damaged slime coat, along with proper nutrition:

(1) Aqueous aloe vera gel 5% to about 30%

(2) Sodium carboxymethyl cellulose (cmc) up to about 7.5 g/l (g/L is an abbreviation for grams per litre : – .0265 oz per .28.16 oz) – mucoprotein slime-replacing compound.

(3) Polyvinylpyrrolidone (pvp having the molecular weight of about 40,000 – K-30 available from GAF Corporation) 1.3 to about 25 g/l  (0.046 oz  / 28.16 oz) – mucoprotein slime-replacing compound .

(4) Sodium thiosulfate or asorbic acid 12.5 (.44 oz  to 2.12 oz  /  28.16 oz) to about 60 g/l  – dechlorinator

(5) Ethylenediaminetetraacetic acid (EDTA) 2 g/l (.07oz  / 28.16 oz) – chelating agent.

(6) Tris(hydroxymethyl)aminomethane 0.3 to about 1 g/l ( .001 to .035 oz  / 28.16 oz) – maintain the pH<>Diazolidinyl urea 1.3 to about 4 g/l (.05oz to 1.4 oz  / 28.16 oz) –  preservative

(7) Aloe vera gel (“VERAGEL 1501”) 4 liters (1.057) gallons.

(8) Sodium thiosulfate 1400 g (49.38 oz)

(9) Carboxymethyl cellulose 100 g. (3.52oz)

(10) Polyvinylpyrrolidone 400 g. (14.11)

(11) Ethylenediaminetetraacetic acid 40 g. (1.4 oz)

(12) Tris(hydroxymethyl)aminomethane 17 g. ( 0.6 oz)

(13) Diazolidinyl urea 80 g. (2.8 oz) deionized water enough to dilute to a total volume of 40 liters (10.6) gallons