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Nutritional Diseases of Fish

By Barbara D. Petty, DVM, North Florida Aquatic Veterinary Services ; Ruth Francis-Floyd, DVM, MS, DACZM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida

With so many species of fishes kept in home and public aquaria, providing good nutrition is a challenge. (See also Nutrition in Fish.) A major hurdle is that nutritional requirements are known for only a handful of fish species. Given the vast diversity in fishes, it is dangerous to assume what is good for one type of fish will be good for a similar but unrelated fish. At the basic level, it can at least be determined whether the fish is carnivorous, omnivorous, or herbivorous. However, beyond that, the amount of protein and lipids required is unknown. In general, many fishes have a higher requirement for protein than other vertebrates.

Inadequate nutrition can result in poor growth, deformities, a depressed immune system, hepatic lipidosis, and impaired metabolism. Iodine deficiency can result in thyroid hyperplasia, which has been seen in both elasmobranchs and teleosts. The condition may be caused by an overt nutritional deficiency or may be associated with chronic nitrate exposure and/or the application of ozone (see Aquatic Systems).

Hepatic lipidosis is a common problem seen in captive fishes and can occur for multiple reasons. Examples include starvation, a high percentage of carbohydrates in the diet, a high amount of lipids, and rancidity.

Fish require ascorbic acid, delivered in the feed. Most foodstuffs for fish should be supplemented with a stabilized form of ascorbic acid. Inadequate dietary vitamin C can result in a condition called “broken back disease” by some farmers and hobbyists. Severely affected fish exhibit extreme scoliosis. Less obvious, but detectable on wet mount examination (100×) of gill tissue, is “bent” or deformed cartilage, which may also indicate a history of ascorbic acid deficiency.

Feeding a wide variety of feeds is one way to try to meet the nutritional requirements. Live feeds can be varied with diverse commercially prepared feeds. This practice will also help prevent animals from eating only one type of feed. Fishes that feed at the surface should be fed feeds that float, whereas bottom-dwellers should be fed items that rapidly sink. The feed should be in particles easily ingested by the fish. Flaked feeds may be too large for a small-mouthed fish to easily ingest. Such feeds can be crushed to enable easy ingestion. In contrast, feeding flaked feed to a large fish like an oscar or other cichlid will result in a messy tank, because larger fish cannot easily ingest enough flaked feed to meet their requirements.

When feeding a mixed population of fishes, as is typical in hobbyist tanks, several types of feed items may be required to meet the needs of the fishes. Often, popular bottom-dwelling fishes such as loricariids and Corydoras catfish are expected to survive on the leftovers from the other fish. This is not an acceptable practice, and these fish should receive targeted feeding.

Another consideration is quantity of feed fed. Ideally, adult fishes should be fed 3% of their body weight daily for maintenance. Fry and fingerlings can be fed up to 5% of their body weight daily. Various types of feeds have dramatically different weights. For example, flaked feeds tend to be much lighter than pelleted feeds. Pelleted feeds come in different sizes and can be floating or sinking.

Most aquarium fishes should be fed at least daily, except for carnivorous fish that ingest large meals. These may be fed once or twice a week.

Careful attention must be paid to storage of feeds. Fish feeds usually have a high protein and oil content, which can deteriorate rapidly. Dry feeds should be kept in a pest-proof container and in an area of low humidity and temperature. High humidity and high temperature result in degradation of the diet, promotion of mold growth and potential mycotoxin production, and rancidity. Many feeds can be frozen, which will prolong their shelf-life. A feed container kept at room temperature should be discarded after 2 mo of opening. Commercial feeds stored in the freezer should be discarded after 6 mo. Commercial frozen feeds such as brine shrimp, bloodworms, mysids, glassworms, etc, should be discarded after 1 yr. If thawed, these feeds should not be refrozen.