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Copper isn't only poisonous to algae (including blanket weed or cyanobacteria) at low levels but it's also poisonous to freshwater fish, such as koi, at slightly higher levels. The manufacturers of copper based algae treatments rely on the user dosing the pond to a level that is poisonous to algae but not immediately poisonous to koi.
That's a tricky balancing act to maintain, especially with water changes and top ups that require the small additions of the treatment that will be required to maintain the level necessary to continue killing algae. However this balancing act is achievable and so the manufacturers can "prove" that their treatment has killed algae without immediately having killed any koi BUT....
Even if the level is kept below that which will cause rapid death, copper accumulates in koi tissue especially with long term use. The correct term is that copper is a bioaccumulative poison. Many toxic substances that are taken up or ingested by a living organism are slowly removed or excreted by natural mechanisms but bioaccumulation is a process whereby something is taken up and accumulates in organs or body tissues more quickly than the natural processes can remove it.
Common algaecides such as copper or zinc, if used at a level that is effective as an algae treatment, will be taken up by fish at a higher rate than they can be removed and therefore they will accumulate in tissues or organs where the levels become detrimental. If these types of product are used on a long term basis, the koi will be less healthy and will probably die earlier than they would have done if they hadn't continually been subjected to a poison at a sub-lethal level.
Note: Copper in trace quantities only is an essential mineral in fish diets (and ours) because it is involved in the production and activity of many enzymes so it is frequently found in koi food and I have no problem with that. Trace quantities of copper in fish food refers to amounts typically in the region of 3 mg to 5 mg per kilo of food. These quantities are totally harmless to koi but are also harmless to algae. Increasing the amount of copper to a point where it becomes harmful to algae means that the amount is way above the dietary needs of fish and, at that level, it bioaccumulates and becomes detrimental to fish health.
Copper in the aquatic environment
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Science panel
To be precise, one mg/L (or one ppm) is 1 mg of the substance dissolved in 999,999 mg of water, or 2 mg dissolved in 999,998 mg of water, 3 mg in 999, 997 mg of water and so on.
In any case, the total amount of water plus the dissolved substance should add up to exactly one million units.
However, outside establishments such as science laboratories, where precision is important, it’s easier to think of units of the substance being dissolved in one million units of water.
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Copper is a naturally occurring metal found in the earth's crust and is therefore also generally present in natural surface waters, with cupric ion (Cu2+) as the primary form.
Copper with a 2+ charge is also the form of copper that is most effective for algae and parasite control. In bodies of freshwater, naturally occurring concentrations of copper (Cu2+) range from 0.2 µg/L to 30 µg/L (Bowen 1985).
Koi keepers are usually more familiar with concentrations or levels of dissolved substances measured in mg/L (ppm) which means that one unit of the substance is dissolved in one million units of water. (See science panel →).
The unit µg/L stands for micrograms per litre and is equal to one one thousandth of 1 mg/L. Another way to understand µg/L is that 1,000 µg/L = 1 mg/L.
At trace concentrations, copper is an essential element to virtually all plants and animals, including humans. At slightly higher levels than the normal dietary requirement, copper becomes highly toxic to aquatic organisms and may cause irreversible harm at concentrations just over that required for growth and reproduction (Hall et al. 1988, Eisler 2000, Baldwin et al. 2003). The American Environmental Protection Agency standards for environmental water quality for the protection of salmon and other freshwater species is that copper levels should not exceed 9 µg/L at a hardness of 100 mg/L (CaCO3) which is a typical hardness for koi ponds so the comparison is valid.
Effects on koi
Extrapolation from the EPA recommendations of the effects of copper on salmon that also will cause a similar effect in carp include:
Impairment of the immune response.
Disruption of gill function making respiration less efficient.
Disruption of the osmoregulatory system.
Impairment of brain function.
Changes in enzyme activity, blood chemistry and metabolism.
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Science panel
Epithelial hyperplasia:
Damage to epithelial cells, which results in shortening, rounding and fusion of the gill secondary lamellae. Sometimes even the gill primary lamellae are affected.
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Toxicity studies on carp
To study the toxic effects of copper on carp, toxicity tests were carried out by a team of researchers from the Faculty of Agriculture in Yugoslavia. After a 14-day period of exposure to five concentrations of copper sulphate (0.25 - 4.0 mg/L CuSO4), changes in gill structure were investigated.
The results of biochemical analysis were confirmed by histopathology (microscopic examination of tissue). Lesions on the gills were observed which included epithelial hyperplasia (see science panel →), curling of secondary lamellae, and changes in chloride cells. These lesions severely affect the structure and function of the gills and reduces the surface area available for the take up of oxygen and removal of carbon dioxide from the bloodstream. The severity increased with increased copper concentration.
Effects on the biofilter
Copper is also toxic to the nitrifying bacteria in the biofilter. At a high level such as 0.3 mg/L, copper sulfate Cu2+ inhibits ammonia and nitrite oxidation. In less technical terms if the level of dissolved copper becomes high due to over enthusiastic use of a treatment it halts all stages of the nitrogen cycle. Increases in ammonia or nitrite levels in the system may occur if copper is used as a pond treatment. By contrast, bacteria that can cause disease in fish are much more resistant to copper, with some only inhibited or killed at free copper levels as high as 1.25 mg/L (Cardeilhac and Whitaker 1988).
One of the worst ways to introduce copper into a pond with the aim of controlling algae growth is by the use of devices that electronically dissolve copper into pond water. I’ve seen many of these types and I’m honestly appalled, not just by the fact that they are sold in the first place but, even more so, by the lack of any proper control of the output or levels of dissolved copper that can result from their use. Don’t be fooled by advertising statements that say the device only electromineralises the water or that it adds natural minerals that inhibit algae growth. Many natural minerals such as arsenic are poisonous or harmful to aquatic life. Simply stating that a mineral is “natural” is no guarantee that it is safe to add to a koi pond.
Another misleading statement proudly stated by one manufacturer is that the level of copper their device causes to dissolve into the pond is below that of ordinary tap water so, if it’s safe for human consumption, it must be safe for koi. The maximum safe level of copper that is allowed in UK drinking water supplies is 2,000 µg/L. The universally accepted maximum values of copper that should be allowed in a koi pond is 5 µg/L in soft water or 15 µg/L in hard water. 2,000 µg/L is between 133 times and 400 times the level of copper that must not be exceed in a properly managed koi pond!!
Sales phrases like those above are just designed to hide the fact of what these types of electronic algae killing devices really do. They add copper to the water and if the manufacturers of the devices are fully confident that copper kills algae but, somehow, doesn’t harm koi why don’t they say that openly?
Right of reply
Despite my undisguised anger that these devices are being sold to unsuspecting hobbyists I’m fully aware that manufacturers of these electronic devices will disagree with me about the dangers to koi posed by their devices so I will give them the right of reply. If any manufacturer wishes to dispute what I say and assert that their device is harmless to koi, simply contact me via my contact page (link below) and we will have a debate. You may produce whatever research, facts or data you feel supports the safe use of your device but, in return you must answer the series of questions I will ask.
I promise to publish here the full email debate without alteration or editing. That is my challenge.
Pseudo science isn’t real science and cuts no ice with me
One manufacturer did try debating the merits and safety of their device on a forum that has since closed. He quoted lots of sales talk but was unable to give any coherent answer my list of questions which grew in size after each of his attempts to bamboozle those who were looking in. He eventually gave up and disappeared when he realised that sales talk was no answer to scientifically based questions and reasoning.
Contact me
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