There are several causes of kidney disease (also referred to in medical terminology as renal disease) in aquarium fish.
Numerous bacteria can infect fish, and if the infection becomes systemic (spreads throughout the body) the kidney is usually affected. The kidney may become enlarged and there may be an accumulation of fluid in the abdomen. Systemic bacterial infections often cause external signs as well, including skin ulcers and hemorrhages. Diagnosis is made through a bacterial culture or immunological tests. An antibiotic sensitivity test should be performed to determine the appropriate antibiotic. Many bacterial infections can be prevented through proper husbandry such as maintaining high water quality, providing good nutrition, and preventing overcrowding and other sources of stress.
Pond goldfish are susceptible to an infestation with the parasite Hoferellus carassii, formerly known as Mitraspora cyprinid. The disease is often referred to as 'Kidney Enlargement Disease' (KED) or 'Kidney Bloater.' The parasite causes abdominal enlargement due to swelling of the kidneys and ureters. The disease is fatal. The life cycle of this parasite is generally one year in length, with spores (immature forms of the parasite) being released into the water in the spring. Fish become infected in the summer, generally start showing signs of disease in the fall, and usually die the following spring.
The diagnosis of Kidney Bloater is made by finding the spores in affected tissues. There is no effective treatment. Ponds with fish that have become infected with Hoferellus carassii should be drained, disinfected, and restocked with known uninfected fish.
Although high levels of copper in the water usually affect the gills, the kidney and liver may also be damaged. High levels of copper in the water can be detected through the use of commercial test kits. Fish affected by high levels of copper should be removed to another system and water changes should be performed on the original system. Polyfilters and chemical copper removers such as CuprisorbTM by Seachem are recommended for removing excess copper.