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.