Copper is toxic to Koi yet I know a lot of Koi keepers who have copper plumbing in their homes and don't seem to have any problems.
The LC50, which is used for aquatic life, stands for a lethal concentration of 50%. This means the concentration (in the water) that has a 50% chance of causing death. A lower LC50 means the substance is more toxic.
The LC50's for copper to freshwater fish are generally less than 0.5 parts per million (ppm), and can be as low as 0.006 ppm.
Exposure to chronic/sub-lethal levels (0.02 - 0.2 ppm) of copper reduces survival, growth, and rate of reproduction in a variety of fish species1. The most sensitive species to copper are flathead minnows and golden shiners. In some studies they are 30 times more sensitive than other species. Koi, being members of the Carp species, are very closely related to flathead minnows and golden shiners, and are very sensitive to copper.
After Mercury, Copper is one of the most toxic metals to Koi. Copper ions precipitate gill secretions, causing death by asphyxiation. Gills of adult freshwater fish, which typically make up 50% of the total body surface area, are more thin and delicate than the skin epidermis. The gill surface may also have a substantial net negative charge, therefore having a high affinity for cationic metals.
One pond owner's Koi caretaker added 0.20 ppm of elemental copper in a quaternary ammonium chelating salt to get rid of algae. (He had done so the year before, and all the Koi had become very sick, but all recovered.) This time 14 of the 50 or so Koi died by asphyxiation. Most of the deaths were to the larger Koi. The pond's GH (general hardness of calcium and magnesium) was 90 ppm which is relatively soft. Its KH (carbonate hardness) was also 90 ppm. Many of the plants had not been put into the pond yet. They were still in the greenhouse. The pH was 8.6, the salinity was 0.1%.
It is somewhat of an enigma. Why can one pond handle more copper than another before killing Koi?
It turns out that many mechanisms are at work. When you add copper to a virgin pond some of it is absorbed out of the water by plants and sediment. Plants can absorb up to 100 mg/kg of dry weight. Desorption from sediment back into the water depends on pH and salinity. If you add the same amount of copper later, after the plants and sediment are already loaded to capacity with copper, more copper will be concentrated in the water and this time can cause death.
Lower pH's increase the toxicity to Koi via several mechanisms: first, the greater acidity increases the Copper ion concentration in the water; second, it causes a loss of electrolytes, and interferes with ion regulation by Koi.
Salt has a high complexing ability and reduces the toxicity of copper to Koi. So higher salt levels are beneficial.
Hardness also reduces the toxicity of copper to Koi sometimes by 20 times or more. So harder water is beneficial. One researcher found the 96 hour LC50 for copper for rainbow trout in soft water was 0.02 ppm, and in hard water it jumped to 0.52 ppm, a difference of 26 times. To see how to increase the hardness in your pond click here: alkalini.htm
1 - For more information on this subject click on: http://www.science.mcmaster.ca/Biology/4S03/COPPER.HTM