What causes swim bladder disease? 

Q: One issue that I find really difficult to remedy is that of swim bladder. Thankfully, it is a rare occurrence but I have never successfully treated a fish with this condition and eventually the fish passes away, which is very frustrating. Please could you provide some information on this problem, what causes it and how it can possibly be treated?

PAUL HAGGIE, VIA EMAIL

A: Peter replies: The swim bladder, also known as the ‘gas-bladder’, is basically a gas-filled bag that lies within the fish’s body cavity. It enables the fish to maintain neutral buoyancy in the water, such that the fish neither floats nor sinks. The fish’s soft tissues and skeleton are denser than water, hence a fish would sink without an effective swim bladder.

In most fish the swim bladder is a single chamber. In some, notably cyprinids, which includes the barbs, rasboras, minnows, goldfish and Koi, it comprises two chambers that are connected by a narrow duct. Three swim bladders are found in many catfishes. In some fish species, the swim bladder also plays a role in respiration, or in sound production, or sound reception.

Swim bladder problems can affect a whole range of fish species, both freshwater and marine. 

Goldfish, both long-bodied and short-bodied varieties, seem particularly prone. 

Typically, only a single fish is affected. One key sign of a possible swim bladder problem is when a fish floats at the surface or sinks to the bottom. Often, this switch to abnormal buoyancy occurs suddenly. In other cases, the fish may swim in an unnatural position, such as a head-down pitch, or listing to one side.

If the swim bladder is over-inflated then the fish will rise to the surface.

In some cases, the fish will float on its side or upside-down, with a proportion of the body exposed above the water-line. In addition to being stressful, the air-exposed skin will dry out, risking ulceration and infection. Conversely, if the swim bladder is under-inflated, collapsed, ruptured, or filled with fluid, the fish will tend to sink to the bottom, risking abrasion of the belly region on the substrate.

Identifying swim bladder problems

Importantly, not all abnormal swimming behaviours or buoyancy problems are due to a defective swim bladder. Some infectious diseases caused by certain bacteria, viruses and even some parasites, or water quality problems such as toxic levels of ammonia, or contamination by insecticides or pesticides, can damage the fish’s central nervous system, resulting in uncoordinated swimming behaviour. Excessive accumulation of gas within the gut, perhaps due to a gut infection or digestive problem, can also cause the fish to float. 

Outward signs rarely give a clue to the underlying cause of the problem. The most reliable way to investigate a possible swim bladder problem is with an x-ray but this of course means taking the affected fish to a vet. The sedated fish is held briefly out of water for radiography: the gas filled swim bladder is clearly defined as a dark, ‘radiolucent’ area. Radiography enables the vet to visualise the size and position of the swim bladder, and whether it has collapsed, enlarged, been displaced, or is abnormally filled with fluid. 

Even with radiography, it may not be easy to accurately diagnose the underlying cause of a swim bladder problem and unfortunately many cases are untreatable. 

Because the causes are so diverse, there can be no single treatment for this condition. The following are all possible causes:

● Certain bacterial, viral and even some fungus-like infections can affect the swim bladder. Some bacterial infections, such as those caused by mycobacteria, have been associated with fluid accumulation in the swim bladder. Where a bacterial infection is suspected, this might respond to an anti-bacterial treatment or, ideally, antibiotics from a vet. Viral infections
of fish are virtually impossible to treat. In other cases, the swim bladder itself may be uninfected, but could be displaced or distorted by a nearby organ that has become diseased.

● The swim bladder may be abnormal due to a genetic condition or developmental problem.

●  Abnormal body shape can also make the fish more prone to swim bladder problems.

●  Certain internal tumours may grow to the point where they compress or displace the swim bladder, resulting in buoyancy issues. Unfortunately, such tumours are untreatable and usually end up killing the fish.

●  Water problems, such as high nitrogenous waste levels or a rapid drop in water temperature (cited in the case of pondfish) have been linked to swim bladder problems, however these claims are mostly anecdotal and require further investigation.  

● There is circumstantial evidence that the diet or feeding behaviour, or constipation, could be responsible for some swim bladder problems, particularly in goldfish and other cyprinids, as the swim bladder in these fish is connected to the oesophagus by a thin tube. Surface feeding, resulting in air-gulping, has been considered a possible cause of gas accumulation in goldfish, leading to floating. There are many claims that floating problems in goldfish have been improved by starving the fish for a few days to help clear the gut, or feeding crushed fresh/cooked peas to purge the gut, and these harmless treatments are definitely worth a try.  

Deep-water fish that are quickly brought to the surface may suffer from rapid expansion of the swim bladder and related problems, with potentially lethal consequences. This is an issue with the collection of reef fishes for the aquarium trade, such as Yellow tangs and particularly those from deeper water. It can be prevented by ascending the fish in stages, in decompression stops. In the absence of decompression, some collectors puncture the swim bladder with a needle to prevent over-inflation.   

Treatment

Finding the right treatment is tricky where the underlying cause is unknown, as is often the case. If goldfish or other cyprinids are floating, try a few days starvation or purging them with peas in case it’s a gut problem. Failing that, seek veterinary advice with a view to radiography and perhaps antibiotics if a bacterial infection is suspected.

Over-inflation of the swim bladder may be treated by a vet in some cases by venting off excess gas using a syringe needle. But often this provides only a temporary solution as the swim bladder may over-inflate again. The surgical insertion of weights or fitting of external floats (typically done with goldfish) have corrected some cases of floating or sinking problems, but of course these methods don’t address the underlying cause.

In general, always check and remedy any water quality issues, regardless of whether they may be a cause of the problem. A visit to the vet is best, but if this is not an option then try medicating the fish with an antibacterial remedy from an aquatic shop just in case a bacterial infection is to blame.  

Sadly, many cases fail to improve and euthanasia may be the kindest option.