Nutritional Disease

Nutritional disease has been defined as a deficiency, excess, or improper balance of the components present in the fish’s diet. Nutritional diseases are uncommon in wild populations of rainbowfishes. Although environmental factors may limit food availability, wild rainbowfishes generally have the opportunity in their natural environment to acquire adequate levels of all their nutritional requirements. The same cannot be said for captive rainbowfishes. There are many different types of foods available for feeding rainbowfishes, including commercially prepared pelleted and flake foods, live and frozen foods (e.g., brineshrimp, daphnia, bloodworms, etc.) and homemade formulated foods. Nevertheless, the nutritional value of these foods is highly variable. Rainbowfishes offered nutritionally deficient diets are often more susceptible to infectious conditions.

Most commercially prepared pelleted or flake foods are comprised of a combination of macronutrients and micronutrients. The levels of these nutrients can vary between brands and formulations. However, artificial diets produced by major manufacturers and intended as a complete source of nutrients are usually of high quality and it is rare that a particular mixture will be responsible for a nutritional disease. These commercial diets are more likely to cause problems associated with excessive nutrients from overfeeding. However, when feeding homemade food diets, only one type of live or frozen food, the possibility of a nutritional deficiency or imbalance is much higher.

Rainbowfishes have different nutritional requirements based on size, age and water temperature. Obviously, no single food will meet their needs at all life stages, and the best way to ensure that they are getting a well-balanced diet is to feed them as wide a variety of food as possible.

Rainbowfishes in captivity are susceptible to both macronutrient (e.g., protein, fat, carbohydrate) and micronutrient (e.g., vitamin and mineral) deficiencies or imbalances. Among the micronutrients, any of a wide range of components can exert an effect, especially in fast-growing, younger fish. In terms of macronutrients, most problems are associated with the lipid (fat) component of the diet. When more protein or carbohydrates are added to a diet than is needed for growth and other bodily functions, the excess will be excreted as waste or stored as body fat. Excessive fat in the diet can lead to hepatic lipidosis (fatty liver disease). Rainbowfishes with this disease may display a range of clinical signs, including inappetence, lethargy, internal granulomatosis, darkening skin colour, poor growth, anorexia, obesity, and fertility problems. The significance of obesity in captive rainbowfishes is uncertain, but hepatic lipidosis or mycobacteriosis should be considered in cases of prolonged abdominal distension in rainbowfishes.

Hepatic lipidosis is a serious and often fatal disease of cultured fish species and has primarily been studied in aquacultured food fish species. In the aquarium hobby however, hepatic lipidosis has been reported in African cichlids, goldfish and rainbowfishes, and can be the result of too much fat in their diet, overfeeding or feeding rancid or vitamin E deficient foods. It affects the function of the liver and can result in death of the fish.

Lipid metabolism is mainly regulated by the liver including both the synthesis and degradation of fatty acids. Several enzymes regulating these pathways show varying affinities for the different fatty acids available in the liver, and thus the imbalances in the dietary fatty acids could modify the function and morphology of this organ (Caballero et al., 2004).

Vitamin E is the major lipid-soluble antioxidant in the cell antioxidant defensive system and is exclusively obtained from the diet. The primary function of vitamin E is to maintain the functional integrity of cellular and subcellular membranes, by preventing lipid peroxidation of fatty acids. Oxidative processes during the storage of fish food can destroy vitamin E and other antioxidants. This does not appear to be a problem however, if fish foods are properly stored. Even high-quality foods are susceptible to nutrient loss during storage, especially if these foods are stored improperly.

Sporadic cases of hepatic lipidosis have been reported from a variety of fish species in home and public display aquaria and have been attributed to the bacteria Mycobacterium, protozoans and nutritional imbalance. Several cases have been diagnosed in goldfish (Carassius auratus), and Noga (1986) reported a case from a private breeder involving several African cichlids. He found a diet-associated granulomatous condition in the cichlids that were being fed a tropical fish flake food. No pathogen was isolated in either of these cases. After a change of diet, the disease disappeared from the cichlid population. Noga further implicated diet as the immediate cause by feeding the suspect diet and a known -quality diet to two groups of fish from a different source. Lesions developed only in fish fed the suspect diet.