By Jessie M Sanders, DVM, CertAqV
Bony fish have a specialized organ called a swim bladder. The purpose of this organ is to contain oxygen and gases to maintain neutral buoyancy at the fish’s desired depth, similar to a diver’s buoyancy compensation device (BCD). These fish, called physostomes, fill their swim bladder with oxygen by gulping air at the water’s surface, where it quickly passes through a pneumatic (air) duct to the bladder. In physoclist fish, a specialized gas gland that pulls gases from the blood keeps the bladder filled. The swim bladder is surrounded by a tough outer membrane and lies just under the spinal cord in the coelomic cavity.
In addition to aiding posture and swimming ability, some fish use their swim bladder for sound production and detection. This organ is very significant in the overall health of fish. However, it is not exempt from disease and dysfunction.
(See a fish’s anatomy with internal organs illustrated here.)
What Causes Swim Bladder Problems?
Many contributing factors can cause swim bladder disorders. One of the most overlooked components is water quality. Poor water quality can result in sudden and chronic stress in fish. Stress causes disruption in regular homeostasis, which can result in negative or positive buoyancy disorders. If your fish presents with a buoyancy disorder, water quality should be checked immediately and corrected if necessary.
If your fish needs to be seen by a doctor, make sure your veterinarian is comfortable working on aquatic animals before you proceed. Or, to find an aquatic veterinarian near you, consult the following databases:
American Association of Fish Veterinarians
World Aquatic Veterinary Medical Association
The best way for your veterinarian to evaluate the swim bladder is by taking an X-ray. X-rays can show the positioning and size of the swim bladder very clearly. It can also show if there is any fluid in the swim bladder, which is not a normal condition. Swim bladders can become displaced due to disease processes, which will be easily seen on X-rays.
Swim Bladder Disorders in Goldfish
Most commonly, buoyancy disorders occur in Goldfish (Carassius auratus). Goldfish are physostomous, having an open connection between their esophagus and swim bladder. This makes disorders of buoyancy more complicated. Due to their round body shape, and in the case of some fancy varieties, a very curved spine, swim bladder disorders are not uncommon. Sometimes diet is the cause, with excess air entering the gastrointestinal tract during feeding time. Switching to a sinking or neutrally buoyant diet may help correct mild disorders by keeping excess air from entering the duct to the swim bladder.
However, even with diet modification, swim bladder disorders may not be easily corrected. It is always recommended that owners discuss their options with a veterinarian before attempting any buoyancy compensation devices, such as floats or weights. Tying foreign structures to a fish’s body can have catastrophic effects to its skin and mucus production. Any sort of external device will not provide a long-term cure.
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Figure 1: Goldfish with caudally displaced swim bladder secondary to polycystic kidney disease
Figure 2: Lemon, left, and Rusty, right, two fancy goldfish with an externally placed flotation device
Figure 3: Rusty’s X-ray showing a compressed and displaced swim bladder
Swim Bladder Disorders in Koi
Koi (Cyprinus carpio) are also prone to swim bladder disorders. Due to their larger size, special considerations must be taken when trying to X-ray a Koi. Koi with spinal deformities or neurologic damage may have secondary changes in their swim bladder. Swim bladder size and shape can be slowly altered over time to compensate for decreased mobility. These changes, which may become permanent, will allow a Koi with less mobility to survive in its home environment.
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Figure 4: A koi with an enlarged cranial swim bladder secondary to spinal damage
Swim Bladder Disorders in Cichlids
Cichlids are another group of fish prone to swim bladder disorders. They can present either positively or negatively buoyant (i.e., higher or lower than a normal water depth). Similar diagnostics as those described above should be performed to determine the cause of the swim bladder abnormality.
Home Treatment for Fish with Swim Bladder Disorder
Depending on the cause, swim bladder disorders may be temporary or permanent. If your fish has a permanent swim bladder disorder, they can still live a full and happy life with some lifestyle modifications. With positively buoyant fish, some of the fish’s body can spend too much time above the water’s surface, making it important to keep their skin moist. Do not cover the top of your tank to keep your fish submerged. This will result in decreased oxygen diffusion. Ask your veterinarian what can be applied to fish skin to protect it from air. Negative buoyancy disorders, with a fish spending too much time close to the bottom of the aquarium or pond on its side, belly, or head, will need to be controlled with a clean, non-abrasive substrate, such as glass stones. It is critical that these tanks be kept very clean.
Fish with compromised swimming ability will need help eating. With any buoyancy disorder, you will need to introduce hand-feeding. Be patient and try some tasty treats, such as small bits of shrimp, to get them started. Once they have gotten the idea, go back to their regular diet. Fish are smart and will catch on to the new routine quickly. When hand feeding, do not grab your fish! Bring the food to them in whatever position works best for them.
Preventing Swim Bladder Disorders
Buoyancy disorders in fish can be difficult to decipher and may have no permanent solution. If you have a fish that is starting to have problems swimming, check your water quality first. Water quality is often overlooked with swim bladder disorders. With physostomous fish, try a sinking or neutrally buoyant diet to keep excess air from getting into the swim bladder.
If the swimming problem persists, consult your local aquatic veterinarian to help set up X-rays to evaluate the swim bladder. Once the problem has been diagnosed and discussed, make a plan with your veterinarian for your fish’s future. Fish can live long, happy lives with swim bladder disorders, it will just require a few changes to your tank and regimen.
Lewbart, GA. 2015. Swim Bladder & Buoyancy Disorders of Ornamental Fishes.
American Association of Fish Veterinarians. Conference Proceedings 2015.
Roberts, HE. 2009. Fundamentals of Ornamental Fish Health.
Internal Organs of a Fish, Sharon High School via Wikimedia Commons
Goldfish and Koi images provided by Dr. Jessie M Sanders, DVM, CertAqV
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