Copper sulfate

Though copper is an essential trace element for plants (it's involved in photosynthesis) and animals (it's in every molecule of hemoglobin, for one thing), in excess or free in ionic state it becomes toxic.
 
Though copper chemistry involves several valence states, the organic forms are all in the common divalent copper (Cu++), which is quite soluble.
 
Copper sulfate (technically it's cupric sulfate, CuSO₄.5H₂0, traditionally called "blue vitriol") is an algicide and is also used in agricultural sprays against insects and some fungi.
 
Years ago fishkeepers would blithely put sheets of copper "the size of a postcard" temporarily into tank water to control hydra! Thirty years ago, I was maintaining "harmless" low levels of copper sulfate, thinking to "prevent" parasite infestations. I would never do that now. But old ways die slowly: it's not that long ago I noticed a web forum poster confidently recommending leaving a penny in the tank as a parasite preventative; nevertheless, the use of copper as an aquarium biocide has decreased in the last decade. (So has the use of copper in a penny.) 
 
Even marine aquarists became increasingly ambivalent in the 1990s. Columnist Scott Michael declared in Aquarium Fish, Dec 2001, "Although in the past... I've regularly recommended the use of copper, I'm beginning to believe we kill more fish with copper than we save."
 
Copper and plants. Copper is also very toxic to some plants, though others are more tolerant. The "hardwater" plants, such as Vallisneria, tend to be sensitive to copper, even though all plants scavenge minute quantities of copper as a micronutrient, but at concentrations that are measured in parts per billion, not parts per million (mg/L).
 
Copper and invertebrates. Besides hydra, copper kills snails. In fact copper is very poisonous in the aquarium to all kinds of invertebrates, as any reef keeper will tell you. Freshwater invertebrates are even more sensitive to copper than marine inverts. In freshwater, apparently "inconsequential" levels of copper can destroy the infusoria and copepods on which fry feed. So the copper sulfate that acts as an algicide also suppresses the zooplankton that keep algae under control. Blackworms are quite sensitive to low levels of copper. Copper may even depress nitrifying bacteria; at the University of Florida's I.F.A.S. site, the "Introduction to fish parasites" (p.5) cautions you against running copper sulfate through your biofilter.
 
Copper and vertebrates. Vertebrate sensitivity to copper is variable. It is all but non-toxic to birds. Since soluble copper salts such as copper sulfate are not poisonous to us, either — in the US EPA sets a maximum contaminant level of 1.3 mg/L, and cupric sulfate is included in baby formulas — we tend to underrate copper's toxicity to fishes. In fact, your tapwater may contain copper levels, perhaps leached out of copper plumbing, that are harmless to you but toxic to fish; dissolved copper in concentrations of 0.2 ppm will kill some fishes in 24 hours.
 
Molar toxicity and pH. After mercury, copper is the most toxic of the heavy metals. When adjusted for mass and compared molecular weight for weight, in the analysis called "molar toxicity," copper in fact is more poisonous to fishes than lead. Apparently some species of fish are more sensitive to copper than others. In common with plants from soft waters, softwater fishes like tetras tend to be more susceptible. There's a simple connection here. The toxicity of copper is tied to the pH of the water. Copper's toxicity depends on free Cu++ ions. While it remains free and "bioavailable," copper is quite reactive, and poisonous. In water copper is quickly chelated (chemically bound) to organic molecules, like humic acids or amino acids, or complexed to anions (negatively charged ions). It can be adsorbed onto fresh activated charcoal, or to colloidal clay and particulate floc, either in the water or in the substrate. Organic molecules such as humic substances and amino acids act quickly; even low levels of dissolved organics efficiently scavenge Cu++ from the water.
 
The pH factor makes free copper unstable. As alkalinity increases, copper ions tend to bind to calcium carbonate and precipitate out, so that the concentration of available Cu ions in the water decreases. The green skin of the Statue of Liberty is largely a copper carbonate. But carbonates are part of the pH buffer; if the pH drops at a later time, carbonates will tend to dissolve into bicarbonates, and the toxic copper ion gets set free again.
 
So a therapeutic level of copper in water at a higher pH level can be a lethal dose at lower pH values. And very soft water renders copper more undependably toxic and less stable, for pH values can shift in lightly-buffered water. If the pH dips below 6.0, even some adsorbed copper can become soluble and toxic again. So, that effective level of medicinal chelated copper, which had been chemically bound during a forgotten medicinal treatment long ago, can come back to haunt you, if bio-acidification should slowly lower your pH.
 
That's why I feel that copper sulfate and other copper compounds are generally unsafe to use in soft, unbuffered waters, where the pH could drop over time.
 
Marine fishkeepers, as long as they aren't concerned with invertebrates, can use copper compounds with more confidence. So can you folks with highly-buffered African Rift Lake cichlid tanks. The University of Florida site cautions, "Never use copper sulfate in water that has a total alkalinity of less than 50 mg per liter. The concentration of copper sulfate to apply is often calculated by determining the total alkalinity of the water and dividing that number by 100. For example, if the total alkalinity of the pond is 100 mg/L, then [use] 100/100 = 1 mg/L copper sulfate. Do not use copper sulfate if the total alkalinity is less than 50 mg/L. If you have never used copper sulfate, contact an IFAS extension aquaculture specialist for assistance. Use of copper sulfate may lead to severe oxygen depletions, therefore, emergency aeration should always be available. Use of copper sulfate during hot weather, or when algae blooms are dense, is strongly discouraged. Remember, if you do not know the alkalinity of your water and can not measure it then do not use copper sulfate."
 
Though you'll still find a certain amount of "pro" and "con" about copper sulfate that's been used successfully or unsuccessfully in aquaria, rarely will you hear the one piece of information you need to know, in order to judge the particulars for yourself— the pH and the alkalinity ("buffering")! Until you know the pH of someone's water, don't let yourself be influenced by reports of their success or failure with copper.
 
So, when you put activated carbon in the filter at the end of a medication regime that has included copper, it might be wise to temporarily lower the pH. You could use CO₂ diffusion, if you're set up for that, or merely incorporate some peat in the filtration. The temporarily-lowered pH will free the copper ion so that it can be adsorbed to the carbon or the peat. Fresh activated carbon will remove some of the free ionized copper, and PolyFilter may be even more effective in stripping it from the water. Be sure to discard any filter media that have been used to adsorb copper.
 
Other forms of copper. Seachem claims to avoid these perils with their medication Cupramine, a buffered copper solution. Seachem says it is neither copper sulfate nor a chelated form of copper. According to Seachem, though Cupramine is stable in the aquarium, it's fully ionic, effective at low concentrations and doesn't get precipitated in filter media or the biofilm, to reappear and vex you at a later date. I get this from Seachem's own website. I haven't used Cupramine myself, but then my water is unusually soft.
 
"Zycosin" is a proprietary name for a copper salt used in connection with a patented Ich remedy. I googled the web in vain: "Zycosin" is not otherwise used as a chemical designation anywhere. A rival marketer avers that the trademark name originally designated a chelated silver salt, rendered water-soluble, but that after the price of silver skyrocketed in the 1980s, the name was transposed to a similarly chelated "soluble copper salt". Corporate chemistry!
 
Links. The toxicity of copper is important to understand; the government of Alberta department of Environmental Protection sets it out for you in a "Water Quality Guideline for the Protection of Freshwater Aquatic Life".
 
In the 1980s, the U.S. Army Corps of Engineers was still using tons of copper sulfate annually to kill algae and plants in Florida's reservoirs, though it was known to be the second most toxic to non-target organisms, especially plankton and invertebrates, of any EPA-permitted herbicide. Early reaction to this indiscriminate use of copper resulted in a good brief introduction to basic copper chemistry in water and to copper's environmental chemistry, directly applicable to the movements of copper and its toxicity in freshwater aquaria: it was Andrew J. Leslie, "Aquatic use of copper-based herbicides in Florida," 1990, formerly  at the FL Bureau of Aquatic Plant Management website. There was a table of copper toxicity for invertebrates and fishes at different levels of hardness. The current succinct summary "Copper considerations" in controlling invasive alien plants at the University of Florida IFAS site is the nearest equivalent. Drew Leslie's reservations about copper killing non-targeted snails in the environment are recalled by Jess Van Dyke, "The Copper question", 2009.