Disinfection and fish

Tools for bio-security

The main concerns, aside from general biosecurity, for pond fish are Spring Viraemia of Carp (SVC) and Koi Herpes Virus (KHV). The latter has appeared in Japan and caused major losses of fish which has meant global Koi producing regions have been affected.

Everything used on a fish site should always be physically cleaned first. We recommend using a solution of Ark-Klens to clean away large amounts of physical contamination.

Working with fish multifilament nets present special problems for disinfection due to the weave. This has been shown to require special efforts, as fish mucus has a way of entering the weave of the net as it moves under the weight of the fish. This can take pathogens with it and then they can be effectively hidden from disinfection. We suggest after use they be dropped into a tank and left overnight in a combination of Ark-Klens and Virkon Aquatic. Virkon Aquatic contains tensides to help the disinfectant penetrate but the special problems of nets bear extra effort. This mixture should be rinsed off before nets are used.

Virkon Aquatic is safer to use than Tamodine-E around fish, as the name suggests the carrier material has been selected for suitability for fish. So our general recommendation is to use Ark-Klens as a cleanser, basic disinfectant and tenside surfactant for routine use. Virkon is effective against all viruses. A good routine would be to use Ark-Klens in net dips/washes and for clean downs, followed by terminal disinfection with Virkon Aquatic.




Water freezes at 0’C, however this is simplistic. If it is moving then it doesn’t freeze immediately, it has kinetic energy associated with the movement, this overcomes the strength of the bonds as ice crystals form. Static water on surfaces ie. boundary layers freeze, then this ice slowly extends into the body of moving water, reducing the volume (and potentially increasing the level of movement in the reduced volume). This ultimately means that the moving water can get to temperatures below 0oC. Ice is less dense than water so it floats, and gradually thickens at its boundary layer in sub-zero temperatures. Salt is often used in ponds and this lowers the freezing point by interfering with ice crystal formation this means water can be even colder.

Fish, like all animals, are mainly water (50-75%). Fish's own tissues will begin to freeze. As their tissue is cooled water molecules in the tissue between cells begin to clump and freeze, the ice crystals damage cells. The science of cryopreservation uses cryoprotectants to minimise freezing and so avoid damage.

Carp are by nature a warmwater fish. However they are extremely tolerant and will overwinter perfectly well in unheated ponds. EFSA (2008) suggests that carp will tolerate down to 2oC - actually they will go lower but some will die through stress or superchill.

Normal behaviour in water at freezing temperatures was said to be that fish became dormant and simply sat there on the bottom, waiting for Spring. More recent observations have shown this to be untrue, they continue to be relatively active, if available they do eat some bottom living invertebrates, but generally they are not eating since their digestive processes don’t operate well at such low temperatures.

Fish have a body temperature which essentially mirrors that of the environment and the temperature at which they would be expected to freeze and die would be the freezing point of their blood plasma. They have been shown to be able to resist this by 1oC in ways not really understood except in the presence of ice formation. The presence of ice appears to encourage the formation of ice crystals in the skin and circulation, resulting in death.

Freezing temperatures in warm blooded animals lead to frostbite, here blood flow to the extremities has been reduced and then goes through an on-off cycling before stopping altogether. At that point frostbite begins with ice crystal formation in tissues, whilst at the same time those tissues are dying of lack of oxygen normally brought by the blood flow.

‘Superchill’ is a phenomenon which kills atlantic salmon in Canada, due to exposure to ice formation. The fish, like the koi in this case can tolerate the temperatures but close proximity to ice formation in the environment causes crystal formation in the skin and ensuing damage similar to frostbite.

Ice crystal formation in tissues rips cells apart, this will be especially acute in gills which have extremely cold water possibly with forming ice crystals in it being pumped across them all the time.




Large amounts of organic matter left in a pond over the winter will decay and in spring when the water warms up it will cause a bacterial bloom, this may kill fish directly or may stress them and lead to secondary infections or upsurges of parasites.

Koi become fairly dormant at about 4-5oC (~40oF). You should not feed as temperatures are falling towards this region. Koi do not feed much below 10oC (50oF). Above 12oC they can be fed two or three times a day.

Even if you don't use it as a routine (which you should), use ACE–HIGH (to enhance levels of "stress alleviating" vitamins) in the Autumn.

Sudden temperature changes can be lethal so provide a deep area (5-6ft) for the fish to move into if possible. Protect the pond using a frame with polythene cover, if the pond is raised consider insulating the sides. Do not bang on the ice to break it. Melt it with boiling water. Use pond heaters to keep an open section.

The addition of 0.5 oz per gallon of salt into the pool over the winter acts to reduce the osmotic stress on fish, although some plants may not enjoy it! Under normal conditions water leaks into fish along an "osmotic gradient". By adding salt (sea salt or PVD salt) to the water at something approaching the concentration within the fish, this gradient is reduced. Salt also tends to inhibit bacteria and parasites. The downside is that it inhibits freezing and so allows water to supercool in very cold conditions.

Change 25-50% of the water in spring, from the bottom, to remove mulm, Use ACE-High in the Spring to top-up levels of vitamins that have been depleted over the winter.

Do not overwinter diseased fish with ulcers outdoors, or the bacteria will start up before the fish's immune system does. Bring them in and treat them.

Fish with carp pox are best isolated if badly affected and treated occasionally with CHLORAMINE T to keep secondary bacterial infections at bay. Recovery from this viral disease is usually spontaneous as the immune system gets going by June/July. Supplementation with ACE-HIGH is well worthwhile.

Routine use of a course of CHLORAMINE-T to keep parasitic protozoa at bay until fish can handle them is well worthwhile. At low temperatures the fish's immune system becomes very sluggish, if there are high levels of parasites they can overwhelm the fish before its immune system has "switched on" for the summer.