Angelfish, also known by aquarium hobbyists as silver fish, are a popular addition to many freshwater tanks. These fish are characterized by long triangular fins extending from narrow, flat bodies. Sometimes an angelfish will develop a swollen or bloated belly. This swelling doesn't necessarily pose a risk to the animal, although it could be a sign of a serious problem. Watch the fish's habits and other characteristics to better determine the cause.
An angelfish with a swollen belly may be carrying eggs in preparation for spawning. Unlike some live-bearing fish, which develop fertilized eggs inside of them, angelfish spawn. This process involves a female laying eggs before the male fertilizes them. If you have more than one angelfish, the pair may be trying to reproduce, and the swollen belly is due to the eggs inside. Lay a flat surface, such as a piece of plastic or ceramic pot, at an angle in the aquarium to provide a place for the fish to comfortably lay the eggs and spawn.
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Like other animals, angelfish can have poor digestion and become constipated. This back-up in its intestines could lead to a swollen belly. All fish are prone to constipation, yet the angelfish's narrow body makes it more susceptible. Along with a swollen belly, constipated fish will stop eating food. It's important to treat the constipation, which could lead to death if left alone. Soak the fish's flakes or dried food in glycerol or castor oil, or feed it mashed peeled peas, which will work as laxatives.
Dropsy is a serious disease that affects angelfish. It is typically characterized by a swollen belly, as well as bulging eyes and scales that look as if they'll pop off. The swelling occurs because the angelfish is taking in and absorbing water faster than it can expel it. The disease can be caused by high nitrates in the water, although it is also caused through bacterial infections. To treat dropsy, put the angelfish in a separate tank and add tetracycline antibiotics to the water. Perform water changes in the original tank to avoid spreading the disease to other fish.
There are a variety of less-common causes of swollen bellies in angelfish, including internal parasites. Hook-worms, fungus and velvet are a few parasites that can infect your fish and cause its belly to swell. Parasites are usually brought into a tank with the introduction of new fish. They can be treated with antibiotics and non-iodized aquarium salt. Other causes of an angelfish's swollen belly could be kidney problems, such as a kidney infection, cysts and lesions. Talk to a fish aquarium professional to discuss treatment options.