Koi swim bladder and water temperature Why have a swim bladder? There are species of fish that live on the ocean bed that do not possess a swim bladder as they can access everything they need for survival on the seabed. Fish have had to adapt to new environments and diets when competition for food threatened their existence, which created the huge diversity in fish species and decided who needed a swim bladder. Carp, for example, are bottom feeders and their wide distribution made a swim bladder important not to regulate swimming as the name suggests but to maintain buoyancy at various depths of water. Carp have to remain upright and buoyant in water and this is achieved by creating a state similar to weightlessness in which the external pressure on the fish is minimal. Without such a system, koi would have to swim continually or sink to the pond floor with overwhelming fatigue and very short lives. In koi, the presence of the swim bladder is vital although it is an organ prone to certain problems. Chill factor In the colder months of the year, chilling of the swim bladder is one of the most common health problems in koi. If the temperature drops quickly in winter, koi may react sooner although it is usually spring when the swim bladder is at risk of chilling. Koi tolerate the cold of winter as it is more constant but many fail to cope with the greater water temperature fluctuations experienced in spring in the UK climate. At night, pond water temperatures can decrease considerably, and very shallow ponds often expose koi to a higher risk of chilling. Some koi are more prone to the problem than others and the same fish can be afflicted every spring. Koi that rest on the pond floor inclined on one side that occasionally swim when the water is at its warmest, usually the middle of the day, are chilled although coping. However, koi that fail to swim at all and remain motionless all day on the pond floor are not coping with the cold, and such Koi can be at risk that the chill might trigger a more serious problem within the swim bladder. Treating the problem Heating the water gradually usually alleviates the problem. If this is impossible, a pond cover with fish safe insulation material can also help. The alternative is to isolate any seriously affected koi in a tank that is slowly heated until the fish behaves normally. Never plunge a fish into warm water as this can make the problem worse. If a fish is found frozen in ice, place it in water a little warmer than the pond initially before gradually applying heat. After 24 hours of warmer conditions most koi will improve although, the temperature at which this is achieved varies fish to fish, so there is no ideal temperature to aim for. Koi with swim bladder chill in an isolation tank need a high level of dissolved oxygen via an airline, a light diet such as wheat germ once the fish is active again and looking for food, and the water quality should be monitored. When water changes are carried out, the salt level should be topped up which is easier with a salt meter. It may be late spring before the tank and pond temperatures equalise and the fish can be returned to the pond. Heating the pond not only alleviates chilling but also raises the immune response which can help to prevent secondary problems within the swim bladder. The use of salt The addition of salt to either the pond or the isolation tank is beneficial for several swim bladder conditions including chilling. By introducing 3 kg of pure vacuum dried salt per 1000 litres of water, 0.3% salinity is achieved which is ideal. It is possible to increase the salt level after 24 hours to 0.6% but this should be carried out gradually. The additional salt is not essential and depends on how well the koi has already improved in the warmer conditions. Before returning koi to their pond, small partial water changes to remove the salt are required unless the pond water contains salt. If salt is in the pond over winter, removing it when the swim bladder issues are finally over is necessary. Salt is for medicinal use only as koi are freshwater fish so long-term use is not advisable. Minor or serious Most koi will recover from a short-term chill; it is exposure to extremely low temperatures for long periods that worsen the condition and create a long-term problem. Temporary swim bladder issues can also occur when koi are moved to a new pond that is far colder or even deeper than the fish is used to. Any type of injury, such as a fish jumping out of the pond, or poor netting and handling techniques can damage the swim bladder. Behavioural changes are then observed as the fish attempt to compensate for the imbalance. Koi may be unable to swim at all, fail to remain upright, sit on the pond floor surfacing only to feed, or develop an exaggerated swimming action. This varied behaviour depends on why the swim bladder is unable to function normally. As there is more than one swim bladder problem it follows there is no single solution. What is wrong It is not possible to diagnose any of the potential malfunctions of the swim bladder without a scan and for most koi keepers that is not at all practical. Even with a diagnosis, treatment requires a qualified scientist with the appropriate experience. One condition that can respond to treatment is re-inflation of the swim bladder although this does not always work in the long term due to secondary problems. Knowing when re- inflation is the appropriate course of action is vital or the procedure can only make the condition worse and this type of treatment should not be carried out by hobbyists. Bacterial infection of the swim bladder does not always respond to antibiotics and improvement can often be temporary if there is still an underlying condition. When a fish is not capable of feeding at all, or swims upside down or on one side every day, euthanasia is the humane option. The long term As long as koi are able to feed and swim upright even if not with their former precision, there is some quality of life. There are thousands of koi with less than perfect swim bladders enjoying long lives. Koi that are prone to chilling are more comfortable in shallow ponds as this exerts less pressure on the swim bladder. Ponds that lack depth and those exposed to chill factor should be covered and, if necessary, heated sufficiently to prevent the extreme low temperatures of winter. Koi are 1°C warmer than their pond water and certain body functions fail as the temperature decreases until, at 1°C, they will be nearing death. Dormant koi often cluster together around a warm spot on the pond floor that a thermometer does not register which often helps them through winter. None the less, in the UK climate, the water temperature can drop suddenly and even freeze.  As a precaution, no pond should ever drop below 5°C.  A thermostatically controlled heater set to protect the pond is a more affordable safeguard than a fully heated pond. Koi are Cultured Carp that are now kept as pets and gradually becoming less hardy with a reduced capacity to tolerate long or extremely cold winters so it is not surprising that koi living in colder climates are more vulnerable to swim bladder chills.