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 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!
Old ways die slowly: just recently I noticed
a web forum poster confidently recommending
leaving a penny in the tank as a parasite
preventative. In fact the use of copper as
an aquarium biocide has decreased in the
last decade. (So has the use of copper in
a penny.) Twenty years ago, I was maintaining
"harmless" low levels of copper
sulfate, thinking to "prevent"
parasite infestations. I would never do that
now. Even marine aquarists are increasingly
ambivalent. 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."
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" cautions you against running copper sulfate
through your biofilter.
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
or mg/L.
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--— E.P.A. 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.
After mercury, copper is the most toxic of
the heavy metals. When adjusted 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 is neither copper sulfate
nor a chelated form of copper. According
to Seachem, though it's 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 my
water is unusually soft.
"Zycosin" is a proprietary name
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!
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: A..J.
Leslie, "Aquatic use of copper-based herbicides
in Florida," 1990, at the FL Bureau of Aquatic Plant
Management website. There is a table of copper
toxicity for invertebrates and fishes at
different levels of hardness.
There are also some useful paragraphs by
Dave Brown in "Copper in aquatic systems," part of his document "Copper in the
environment."
