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The Swim Bladder and how to treat Swim Bladder Disease

What is a swim bladder?

Basically the swim bladder is an organ inside most of the fish species that acts as a buoyancy device so that the fish can maintain its level in the water without having to use up energy swimming. It is a gas filled organ and as it is directly below the dorsal fin of the fish it is very efficient at controlling the central mass of the body weight, this will aid the fish to keep upright. It has several common names including gas bladder, fish maw and air bladder. Some species of fish will even use the swim bladder as a resonating chamber and produce audible noises from it.


Normally swim bladders are only found in ray-finned species of fish, many of the bottom dwellers have evolved to live without a swim bladder while other species of fish such as sharks and rays have never possessed one and need to use the fins to maintain a vertical position and to attain different depths in the water. What is interesting is that sharks and rays evolved on a different route from other fish species about 420 million years ago and it is only since then that fish developed the swim bladder as it is today.

In most species of fish that have a swim bladder the organ consists of two sacs, some species only have the one sac however , these can contract or expand dependant on the pressure around them thus maintaining neutral buoyancy for the fish. The walls of the sacs are very flexible and are lined with crystals making them impermeable to gases and sealing them so that they work efficiently. There are fish that have a pneumatic duct connected to the swim bladder, this provides a connection to the gut, the benefit of this is that the fish can then gulp air into its mouth and pass the gas through to the swim bladder when required. This variation of the swim bladder is normally found in species of fish that are not as advanced as most of the modern species but is very effective. Swim bladders that are connected to the gut are known as “physostomous” swim bladders , swim bladders that are not connected have to top up their gas content by extracting oxygen from the blood, it is believed that this method is far more advantageous as physostomous swim bladders can expel gas too quickly if the fish tries to rise up the water column too rapidly. This can be a great disadvantage especially if trying to escape from predators.

Swim bladders are believed to be an early form of primitive lung,terrestrial creatures that evolved no longer having a use for it simply lost this organ completely over the generations.

Unfortunately the swim bladder can become infected and not work properly, this can be treated over a period of time and is normally resolved without any long term effects to the fish. Swim bladder disease is very common in fancy goldfish and is believed to be a flaw in their selective breeding.

What causes Swim Bladder disease?

The most obvious symptoms of swim bladder disease is when the fish stays at the bottom of the tank unable to rise to the surface or the fish may float at the surface and will be trying to swim around with little success. Sometimes fish can be seen leaning to one side, this is not always due to swim bladder disease, there may be other problems involved.

For years it was debated as to what causes swim bladder disease and it is only recently that scientists believe that they know the reasons, there are a few causes for this ailment:-

  • A viral infection-viruses will attack the walls of the swim bladder causing them to inflame. This will then restrict the efficiency of the bladder and gases may not be released or added to keep the fish at neutral buoyancy.
  • A bacterial infection- this is still debatable as to whether or not this is a cause but if so it would attack the walls of the bladder in the same way as a viral infection.
  • The fishes body shape- This is particularly true with fancy goldfish and other fish that have a globe body shape. The rounded body shape actually squashes their gut under pressure which in turn applies pressure to the swim bladder.
  • Diet- feeding fish constantly with dried commercial food can cause problems in the gut, dried foods tend to soak up water and if the fish eat them as soon as they are added to the tank the food will expand inside them yet again applying pressure to the gut which passes this pressure onto the swim bladder.

How can I prevent Swim Bladder disease?

Keep your water quality to its highest quality, this will prevent any infections from getting to the fish in the first place. Always perform regular water changes and invest in a quality testing kit so that you can monitor the quality of the water in your tank on a regular basis. Getting your water tested at the local aquatic stores is not always the best method.

If feeding dried commercial foods try soaking them in water before adding them to the tank, this means that they are expanded to their full capacity and cannot expand any more inside the fishes abdomen.

Vary the diet with some live or frozen foods to cleanse out the fishes digestive system. Brine shrimp is very good for this, they contain little nutritional value but act like a cleaner inside the fishes gut.

How can I treat Swim Bladder disease?

Normally this condition can be treated with no long term effects on the fish. There are a couple of steps that you can take straight away before reverting to medications. The most common factor for inducing swim bladder disease is constipation and poor water quality so addressing these two problems initially should help clear up the problem. Perform a large water change and then increase the percentage of water changes that you would normally carry out in a certain period, this should increase the quality of the water.

Feed the fish some blanched peas, this acts as a laxative and should relieve any pressure in the gut, starving the fish for a couple of days should have the same effect.

There are commercial swim bladder treatments available but only use these if the above steps show no improvement in the fish.

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