As a veterinarian, my days are typically filled with questions about a range of species that can have a range of different problems. When people find out that I also examine and treat fish, the first two comments, typically, are: “I didn’t know that vets worked with fish” and “How do you administer anesthesia to a fish?”
One of the most common problems presented to me via email or on the phone regarding fish usually start like this: “Doc, I’ve got a goldfish that is floating on its side” or “My goldfish seems to be laying on the bottom of the tank a lot.”
Through experience or “Dr. Google,” most people attempt to diagnose that their fish has a problem with their swim bladder. By the time I am contacted, most people who have experienced this problem have typically tried myriad treatments ranging from adding tea tree oil, Epsom salts or a variety of antibiotics to the water. This article will detail the appropriate steps and treatments… and, as you will find out, quite frequently, swim bladder disease is not a problem caused directly by the swim bladder!
Swim Bladder Structure
The swim bladder, also known as the air bladder or gas bladder, is an organ that develops during the embryonic stage from an out pocketing of the digestive tract. Many fish have two different divisions of the gas bladder. The anterior portion is toward the head and is covered by a thick tissue that does not allow for a great variation in size. It is also firmly attached to the spine, which does not allow for changes in position. The posterior part of the swim bladder is directly behind the anterior, but it is positioned closer to the tail. The posterior gas bladder has a very thin cover and can vary substantially in size depending on the amount of gas within the bladder. The posterior chamber is only connected to the anterior chamber of the swim bladder through a small duct and can change placement due to several different causes. The anterior pole of the kidney typically is found between the anterior and posterior portions of the gas bladder.
There are two main types of fish that are classified by the function of the swim bladder in how gas is taken on or expulsed. Primarily, the swim bladder functions in maintaining normal buoyancy, but in certain types of fish, it can also participate in sound perception (“hearing”) and production (“talking”) and even in oxygen exchange.
Physostomous fish have a duct that connects to the esophagus and allows for gulping or expulsion of air to help inflate or deflate the bladder. This group includes fish such as koi, goldfish and catfish.
Physoclistous fish, such as cichlids and perch-like fish, inflate and deflate their swim bladder by exchanging gases through an intricate network of blood vessels known as the vascular rete and a gas gland.
There are also some types of fish that do not always perfectly follow with the aforementioned classification systems and have an absent swim bladder, such as flounders, or a combination of a pneumatic air duct and a vascular rete, such as goldfish.
In order to understand what is going on when a fish is thought to have a problem with the gas bladder, it must be understood that the term swim bladder disease is slightly mislabeled. Swim bladder disease is a syndrome, which can be defined as clinical signs secondary to a disease process. As stated previously, it is very rarely the swim bladder that is the primary cause of swim bladder disease. Medically, the terms positive or negative buoyancy disorder are preferred.
While buoyancy disorders have not been scientifically researched in great detail, they are typically accepted as having a rapid onset and carry a poor prognosis for long-term survival. With a positive buoyancy disorder, the fish floats on the surface of the water and a portion of the skin will be exposed to air (figure 1). This leads to disruption of the protective mucous barrier and, ultimately, ulcerations and infections of the exposed skin/scales.
Negative buoyancy disorders have the opposite presentation of positive buoyancy disorders; the fish abnormally stays on the bottom of the tank (figure 2). Problems with skin infections and skin ulcerations may still result due to disruption of the protective mucous barrier from rubbing on the substrate. Unless corrected, deterioration is likely from either positive or negative buoyancy disorders.
Examination and Diagnostics
When presented with a case of buoyancy disorder, or any abnormality, you should start by examining the fish’s environment. Compile a detailed history, including when the fish was purchased or bred, if it was wild caught or tank raised, if it showed normal behavior and had been eating previously, and most importantly, what chemicals or drugs were added in attempt to treat the problem. Check water quality, including temperature, pH, ammonia, nitrates and nitrites. Part of this will be assessing the stocking density (amount of fish to the tank or pond volume) and the filtration system. Most test kits will come with pH, ammonia, nitrates and nitrites, but they may also include things such as phosphates, carbonate hardness, etc. The more parameters known, the easier it is to assess the overall water quality.
Nutrition should also be evaluated, as different species of fish can have different dietary requirements. If any environmental parameters are found to be out of the fish’s normal range, they must be corrected.
If all of the environmental components are within normal limits, a veterinarian will sedate the fish and perform a physical exam. The most common sedatives used for this are euganol or MS-222. These products should only be used by professionals, as incorrect dosages could be fatal. Samples taken during a routine exam include skin scrapes, fin clips and gill biopsies. For a skin scrape, a cover slip for a microscope slide is used to gently scrape the skin to obtain some of the protective mucousal barrier for microscopic exam. This allows for many different parasites, including ich, to be properly identified. For a fin clip, a small portion of one or more fins are cut off (as a reminder, under sedation) and examined microscopically. This may also yield more information regarding possible parasite infestation. Gill biopsies are similar to fin clips, in that a small portion of the gills are actually removed under sedation and, again, examined microscopically (figure 3). Gill biopsies provide a plethora of information and can offer a general idea on the fish’s overall health. On gill biopsy examinations, parasites may be found, the fish may be noted to have excess mucous production, which leads to poor oxygen exchange, or the fish may be found to have undergone some incident of trauma.
When a fish is examined for buoyancy disorders, numerous abnormalities may be noted. These could be primary causes of the disorder or secondary causes. Examples of common abnormalities in a fish with swim bladder disease include abnormal posture and position; a possible swelling on one side, which could be the gas bladder itself or a tumor; skin lesions from being exposed to air or rubbing on substrate; and exophthalmos, which is more commonly called “pop eye.” Pop eye can be secondary to trauma, bacterial infections and different types of cancer.
To determine potential causes of swim bladder disease, radiographs, or X-rays, will always be suggested. X-rays are the most useful method, aside from surgical exploration or post-mortem necropsy, for visualizing what is going on in the swim bladder and how it is distributing gas. X-rays take just a few seconds and are taken with the fish sedated and out of water. Each species has a unique arrangement of gas pattern or anatomical pattern of the swim bladder. In goldfish, however, there can even be a difference within the species. For example, a comet typically has an anterior (toward the head) and posterior (toward the tail) chamber of the swim bladder, while a Ranchu or Ryukin has a dramatically reduced, or even absent, posterior chamber. Radiographic contrast agents can even be administered to help differentiate the intestinal tract from other organs. This technique is especially helpful if tumors are suspected. All in all, radiographs can indicate if the disorder is secondary to overinflation of the swim bladder, displacement of the swim bladder secondary to a tumor or mass effect, fluid within the swim bladder, rupture of the swim bladder, and severe gastrointestinal problems.
Positive Buoyancy Disorder
Positive buoyancy disorder, where the fish floats at the surface or on its side, is the most commonly presented form of swim bladder disease, especially in goldfish. Most likely, the cause is overinflation of the swim bladder. Overinflation usually takes place in the posterior chamber. However, the exact cause of the overinflation can be difficult to determine without diagnostics and, as stated previously, in some of the short-bodied goldfish, the posterior chamber may be absent. Overinflation can be caused by parasitic infestation, trauma and even different types of cancers. Oscars are predisposed to polycystic kidney disease. Due to the position of one portion of the kidneys being located between the two chambers of the swim bladder, polycystic kidneys could also stimulate overinflation of the posterior chamber of the swim bladder.
Even if the bladder is not over inflated, displacement of the swim bladder for any reason can result in a positive buoyancy disorder due to alteration of gas displacement. If the air shifts off center, the fish could roll to the side and display abnormal posture. Displacement of the swim bladder typically occurs in the posterior chamber because the anterior chamber is usually firmly attached to the spinal column. The displacement of the posterior chamber of the gas bladder is usually secondary to a space-occupying lesion, such as a tumor or polycystic kidney disease. On occasion, displacement of the posterior chamber can also be a normal variation in certain individuals of fish. However, these fish typically do not exhibit abnormal posture or inability to regulate position in the water column.
Another fairly common cause of positive buoyancy disorder is gastrointestinal diseases (figure 4). Examples include parasites or gastroenteritis. Parasites typically require fecal exam for proper diagnosis and treatment. Different parasites may respond in different ways to medications. Gastroenteritis can be due to stress of shipment, fighting, diet change, etc. Most of the time, correction of the stressor could potentially solve the gastroenteritis. With gastrointestinal disease, excessive gas production may be a secondary side effect and can result in an abnormal distribution of gas with the coelomic cavity. The “belly” of the fish is called the coelom due to the absence of a diaphragm and lack of differentiation between the abdomen and the chest. If excessive gas production is present in the GI tract, the gas-distended bowels can result in a fish swimming “belly up.” However, if fluid or foreign objects are found within the bowels, it can also result in a negative buoyancy disorder, where the fish is found toward the bottom of the tank.
Negative Buoyancy Disorder
Negative buoyancy disorders may be more of a cause for immediate concern and can be associated with a worse prognosis. The poor prognosis could be due to the causes of negative buoyancy disorders, or the fact that many hobbyists wait an extended period of time before investigating the disorder. The most common cause of this problem is fluid buildup in the swim bladder. Fluid can be ingested through the pneumatic air duct that connects the swim bladder to the esophagus while eating. When the fluid displaces air within the swim bladder, the fish has a difficult time maintaining neutral buoyancy in the water column, but typically, it has normal posture. This results in a fish that sits on the bottom of the tank but occasionally swims with effort for a short time and returns to the bottom when it stops swimming. Bacterial infections may also occur within the swim bladder, which can result in fluid production. However, proper diagnosis of a bacterial infection can only be made via pneumocentesis, or sticking a needle directly into the swim bladder in order to obtain a sample for examination. The sample can then be examined under a microscope and even grown in a culture to determine the best antibiotic for treatment. On occasion, the swim bladder may have also ruptured secondary to trauma or an unknown cause, resulting in a complete loss of air within the swim bladder.
Treatment options vary depending on the type of disorder and the underlying cause. However, for each type of disorder, there are environmental changes that can be made. First and foremost, water quality should be pristine and partial water changes should be performed every two to three days. Sodium chloride salt can be added at a level of 2 to 5 grams per liter. However, take caution when adding salt, as some species, such as catfish, are sensitive to salt. Adding salt to the system will aid in electrolyte balance and maintaining hydration. Just make sure to check the salinity of your water prior to adding more and more salt to the system, as salt stays behind during evaporation.
Nutritional support can also play an important role, especially when dealing with gastrointestinal issues. Make sure that the proper diet is being fed. For example, certain species of cichlids require a more herbivorous diet that other cichlid species. If given the wrong protein source, it could result in a condition known as bloat. In some cases of gastrointestinal disease, withholding food for three to four days will allow the gastrointestinal system to work excessive gas out of the fish. Also, feeding crushed green peas could potentially force any excess gas out of the intestinal system. However, a diet consisting solely of peas is not appropriate long term.
After properly diagnosing positive buoyancy disorder due to overinflation of the swim bladder, a veterinarian can remove air by sticking a needle attached to a syringe into the swim bladder and removing a portion of air until the fish can maintain neutral buoyancy. If the swim bladder disease is due to displacement of the swim bladder, surgery may be necessary in order to correct the problem, especially if the displacement is secondary to a tumor.
If there is a negative buoyancy disorder, ultrasound can help identify if there is fluid present in the swim bladder. Under ultrasound guidance, a sample can be taken to see if the cause is due to water or to bacterial infection. If bacteria are present, the appropriate antibiotic can be used to treat the infection. If there are not bacteria, antibiotics should be avoided.
To reduce infection, it is important to keep the fish off the bottom of the tank. There have been several different techniques used by veterinarians and hobbyists to place a fish in a sling or surgically attach a float (figure 5). However, no technique is perfect and some have been known to either pull out or cause skin irritation/infection. If skin irritation or ulcerations occur, topical or oral antibiotics may be suggested.
Swim bladder disease, also known as positive or negative buoyancy disorder, is a complex disease syndrome. In order to attempt to treat the process, it is important to know the exact cause of the disorder. Medications may not always be needed, and treatment is not always successful. Avoid the use of inappropriate or unnecessary antibiotics. If possible, always contact a veterinary professional prior to administering medications in order to assure the proper treatment is applied. A list of veterinarians can be found at vetmed.info and fishvets.org.
Chad Harris, DVM, is the chief of staff veterinarian at North Austin Animal Hospital in Austin, Texas. He attended Ross University School of Veterinary Medicine and sits on the board of directors for the Texas Veterinary Medical Foundation and the executive board for the World Aquatic Veterinary Medical Association. Questions or comments about this article can be sent to firstname.lastname@example.org.