The preferred agent for the eradication of external parasites from marine fish and some freshwater fish is copper.

Still, despite a long history of use, many discordant recommendations of expert aquarists betray a limited

understanding of the basic chemistry of copper in the marine aquarium.

Atoms, the elemental units of matter consist of a positively charged nucleus that is surrounded by a spherical field

of orbital negatively charged electrons. Nuclei display a broad spectrum of attractive force for outer electrons, and

those elements with strongly attracting nuclei tend to accept extra electrons and become negatively charged ions (for

example, chloride), while those with weak nuclei tend to donate electrons and become positively charged ions

(copper). Usually, these ions will neutralize their charges by electrostatic association and form salts or ionic

compounds (copper chloride). Those elements with moderately strong nuclei do not accept electrons but share them

with other moderately strong or weak nuclei, resulting in neutral, or almost neutral, compounds, respectively. When

moderately strong nuclei share with weak nuclei, the electrons occupy the orbital spheres of the strong nuclei more

often than they do those of the weak, resulting in polar compounds with positive and negative zones (for example,

water). Polar compounds or polar segments (groups) of a compound can have electrostatic attraction for other

charged groups or can share a full or partial negative charge with a weak acceptor and, thereby, form coordinate

compounds or complexes. Some negatively charged ions (chloride. bromide. iodide) are also capable of sharing

electrons and forming coordinate complexes.

When soluble copper salts are added to water, the salts dissociate into positively charged cations (the ionic

copper) and negatively charged anions (sulfate, chloride, acetate, etc.), and these ions become hydrated, or

complexed, with water molecules. This is depicted in Figure I. Water molecules are present in excess and, therefore,

effectively keep the ionic copper isolated from its parent anion. Although random interaction with the parent anion

does occur to a limited extent, they rapidly dissociate again. This is indicated by the long and short arrows. The

equilibrium, or the direction of the activity, favors the formation of hydrated ions. In simpler terms, this is akin to taking

a thousand steps forward and one backward. This is what makes the salt soluble! If the equilibrium favored the

association of cation and anion rather than dissociation., the salt would be relatively insoluble.

A solution containing ionized copper that is open to the air eventually loses its copper through precipitation of an

insoluble copper salt. If fish are present, this takes place even more rapidly. This is due to the absorption of carbon

dioxide from air and its release by fish into water, where it dissolves and forms carbonic acid, which dissociates into

hvdronium and carbonate ions. Ionic copper interacts with this carbonate, and here the equilibrium favors association

rather than dissociation, resulting in the precipitation of insoluble copper carbonate. This is what happens to stock

solutions of copper sulfate as well as to copper salts in the freshwater aquarium.

The marine aquarium is different. It contains several anions which interact with ionic copper: chloride, sulfate,

phosphate, carbonate, molybdate, borate, iodide, and bromide. The most important, both because of its high

concentration and strong affinity for copper, is chloride. The chloride anion forms a four-membered complex with

copper (Figure 2) and virtually inhibits any interaction of copper with carbonate. The addition of sodium chloride to an

insoluble suspension of copper carbonate will cause the copper salt to dissolve. Even if copper carbonate were to

form in the marine aquarium, it would readily redissolve. Precipitation of copper carbonate, then, is not a mechanism

of copper loss in the marine aquarium. Because of the strong formation of the chloride-cupric complex, most copper

salts added to the marine aquarium are equivalent, and, provided no filtration is used, all are much more stable than

in fresh water. Why, then, does copper rapidly disappear from solution in the bottom filtered marine aquarium?

As Randy Keith demonstrated in his FAMA (Vol. 3. No. I) article, and as could have been deduced from simple

chemical considerations, copper is precipitated primarily by filtration through filtrants containing magnesium

carbonate. Simple experiments readily demonstrate that copper is rapidly removed from marine solutions by

magnesium carbonate but not calcium carbonate. Any chemist familiar with separation technology knows that copper,

iron, zinc, and other metal ions can readily be removed from solution by adsorbtion on magnesium carbonate. Simple

experiments also readily demonstrate that the rate of loss of copper on magnesium carbonate filtrants is inversely

proportional to the salinity. The chloride-cupric complex, then, inhibits, but does not prevent, the absorbtive loss of

copper. The consequently necessary repeated dosing when treating fish is an inconvenience that results in a

dangerous accumulation of copper in the filter bed. This copper is potentially lethal to fish, makes invertebrate culture

difficult or impossible, and interferes with the biological filter's full potential.

Generally, metal ions form stable complexes and copper forms some of the most stable complexes. The watercopper

complex is so stable that when a solution of copper chloride is evaporated the complex does not break down

chloride, sulfate, or acetate were used. There is no sound chemical basis for asserting that copper sulfate is better or

worse than copper chloride in the marine aquarium. With the water-cupric complex the link to water is through

oxygen, while with the chloride-cupric complex the link is through the chloride. This link, or bond, is usually called a

ligand. For copper, chloride is a stronger coordinate ligand than oxygen. Of other possible ligands, which include

carbon, nitrogen, and sulfur, nitrogen is preferred by copper and, generally, will form the most stable complexes. A

is not suitable for use in an aquarium, it is much more resistant to adsorptive loss than is the chloride complex. The

concentration of the ligand is also a factor in stabilizing a copper complex: Stability increases with increasing ligand

concentration. The formation of ring structures also contributes to complex stability, a good example being the

ethylenediamine tetraacetate complex. This represents a special class of complexes called chelates. Chelates are

very stable structures and those that are water soluble are called sequestering or inactivating agents, because they

effectively isolate metal ions and render them non-reactive. Complexes and chelates may be formed through any

combination of ionic, covalent, or coordinate interaction, but coordinate bonds are characteristic of complexes while

covalent, or ionic, bonds are almost always involved in the formation of stable chelates. Both complexes and chelates

may be negative, positive, or neutral. Negative copper complexes result from coordinate bonding with negatively

charged ions, such as chloride, while positive complexes result from coordinate bonding with neutral but polar,

molecules or groups, such as ammonia or amine compounds. Observe from Figure 3 that, when covalent or ionic

bonds are involved, chelated copper loses its charge or ionic properties. This is characteristic of stable chelates such

as EDTA, which is used in chelated copper products.

Looking now at specific recommendations for the use of copper in the marine aquarium, it is evident that the

moderately resistant to magnesium carbonate adsorption. The most widely recommended copper salt is cupric citrate

or a copper salt combined with citric acid. Since a ring structure is formed this is a chelate, but six and seven

membered rings are not exceptionally stable. Further, covalent bonds are involved, resulting in an essentially

uncharged complex. This makes cupric citrate almost insoluble. It is soluble only under strongly alkaline or acidic

conditions and is useful only in keeping stable stock solutions. Cupric tartrate (copper combined with Rochelle salt)

forms a more stable complex, but is also insoluble under aquarium conditions. In both of these, the copper is

uncharged and unavailable, except as it complexes with chloride. Usually, "chelated copper'' refers to commercial

forms of copper complexed with EDTA as shown in Figure 3. This chelate is both stable and soluble under aquarium

conditions. Unfortunately, it is also sequestered or inactivated, unavailable as a toxic agent either to fish or parasites.

In the EDTA chelate copper is bonded through both covalent and coordinate bonds and is the center of three locked

ring structures, effectively isolating any cupric charge and rendering it totally inactive. If it were not for the competition

for copper from the high chloride concentration of the marine environment, it is doubtful that chelated copper would

have any effectiveness. Acetic acid with a copper salt, or cupric acetate, is sometimes recommended as a more

stable form of copper. This has some merit since acetate does form a complex with copper, but this is only slightly

more stable than the chloride complex. Another recommendation has been to buffer acetic acid with tris

(trishydroxymethylaminomethane). Although this does not appear to be recommended to stabilize copper, but to act

as a buffer, the use of tris is actually one of the better recommendations to stabilize copper that have been made. Tris

is an amine compound and it forms complexes with copper in the same manner that ammonia does, rendering the

copper quite resistant to magnesium carbonate adsorption, leaving it fully charged, neither sequestered nor

inactivated. Although the complex is not totally stable, it represents a remarkable improvement over other types of

copper. There are numerous organic compounds that are capable of forming this type of complex with copper, some

more effective than others, with varying degrees of toxicity. Their use requires thorough evaluation and testing.

Generally, loss of the cupric positive charge, or chelation, or both decrease the effectiveness of copper. One aminecomplexede

copper product that has hsown excellent results in stability, effectiveness, and low order toxicity to fish is

Seachem’s Cupramine™

What is the mechanism of parasite and fish toxicity for copper and how does this relate to marine copper types?

When this question comes up it is generally suggested that copper reacts with sulfhydryl groups, inactivating vital

intracellular enzymes and other proteins. Although copper does inactivate sulfnydryl enzymes and binds proteins, it

does not seem likely from current knowledge of cellular biochemistry that this is a likely mechanism, Only charged

(ionic) copper is effective at the usual recommended concentrations (less than 0.3 ppm copper). Membrane

biochemistry suggests it unlikely that charged copper at such low concentrations could pass through cellular

membranes sufficiently to cause severe intracellular damage. Body fluid analyses of treated fish show no significant

increase of copper during normal treatment. This supports the supposition that ionic copper does not pass through

cellular membranes. It seems more likely that ionic copper acts as a membrane poison, binding to membrane

components, causing disruption of normal membrane functions, and leading ultimately to osmotic shock. This is

Also, the secretion response of fish and their respiratory distress when treated with copper is consistent with thi.~

interpretation.

A comparison of the negatively charged chloride-cupric complex (using copper sulfate) and the positively charged

amine-cupric complex (using Cupramine™) indicates that the amine complex destrovs tomites more rapidly, or at a

lower concentration, than does the chloride complex. When tomites were exposed to 0.2 ppm copper as copper

sulfate, or Cupramine™, those exposed to copper sulfate required close to two hours to show evidence of 50%

disruption or kill, while those exposed to Cupramine™ required about 45 minutes. Fish treated with copper sulfate

showed severe distress after 12 hours at 0.4 ppm and body fluids showed increased copper. Fish treated with

Cupramine™ showed severe distress after 12 hours at 0.9 ppm and little increased copper in body fluids. Recovery

was also more rapid after stressing with amine-complexed copper. This indicates that positively charged amine-cupric

complexes are more effective than the usual negatively charged chloride-cupric complex and that larger or positively

charged molecules (organically bound amine copper) are less likely to penetrate membranes (less toxic to fish) than

smaller or negatively charged molecules (chloride-cupric complex). This also suggests that, possibly, the positively

charged water-cupric complex is the form active against parasites while the negatively charged chloride-cupric

complex is more toxic to fish. This difference between amine-complexed copper and chloride-complexed copper

probably accounts for varying experiences of success and failure in treating fish with copper, since marine aquaria

contain varying amounts of natural chelates and complexing agents that will increase or decrease the effectiveness

and toxicity of copper. Amino acids are good examples of such agents, as thev are natural by-products of the

biological environment of the aquarium, and all amino acids are powerful complexing agents. Many amine-type

organics are also produced by decaying food and other decaying organic matter.

The EDTA-copper chelate passes relatively freely through membranes and fish treated with it show significant

elevations of copper in body fluids. Evidence suggests slow deposition of copper in internal organs, although there

evidence of osmotic shock and the apparent kill rate is at least half that observed with 0.2 ppm ionic copper, although

the eventual tomite mortality is high enough to suggest that for chelated copper the toxic mechanism may only partly

be osmotic shock, the balance being some type of intracellular poisoning. It can only be assumed that the same type

of poisoning takes place in fish, but that mortality does not occur simply because of the greater mass of fish as

compared to parasites.

A key factor in the treatment of fish with copper is probability. Success depends on killing susceptible parasites

before reinfection can occur. The proportion of parasites killed in a given time is dependent on the copper

concentration and the chances of reinfection are inversely proportional to the kill rate and directly proportional to the

degree of crowding and the extent of the original infestation. It is important, then, to avoid crowding and to use as

high a copper concentration as possible without harming the fish. For copper sulfate, a concentration of less than

0.18 ppm has about a 75% chance of success. If maintained for l0 days. Concentrations between 0.2-0.25 ppm have

a 90% chance of success, but are approaching a precarious concentration for fish, about 0.3 ppm. The effectiveness

and toxicity for copper sulfate., however, is highly dependent on the pH and organic content of the water. Increasing

acidity increases toxicity while increasing organic content decreases toxicity. Amine-complexed copper can be used

safely at 0.3-0.5 ppm with virtually 98% chance of success. For heavy infestations of resistant parasites, it can be

increased quite safely to 0.7-0.8 ppm. Chelated copper is ineffective at less than 1 ppm and at 2ppm it has about a

70% chance of success with little danger to fish. Chelated copper can be increased to 2.5-3.0 ppm, but this can be

dangerous for some fish. The length of exposure for any copper type can be decreased from 10-14 days to 6-8 days,

without sacrificing success, if the fish are transferred to another treatment tank after the initial 4 days.

Another area of concern to the marine aquarist is the removal of copper in case of over-dosage or at the end of

treatment. Chloride-complexed copper and citrate copper fall out of solution fairly rapidly by themselves with standard

bottom filtration. This can be accelerated with carbon filtration. For rapid detoxification, chelating agents have been

suggested. If this is done in an emergency, only soluble chelates should be used so that they may be removed by

water change later. Insoluble chelates will only precipitate the copper out of solution and leave it in the filter bed

where it can cause trouble at a later time. It is unwise to use any kind of copper, or copper associated product, that

will deposit copper in the filter bed. Amine-complexed copper can be rapidly removed with carbon filtration. but not

with bottom filtration. Chelated copper cannot be removed with either carbon or bottom filtration. Polymeric

adsorbents and ion-exchangers have also been ineffective in removing chelated copper. The only way to remove

chelated copper is by water change.

Formaldehyde is often recommended in conjunction with copper treatment. It is such a highly reactive substance

that commercial 37% solutions actually contain less than 0.1% free formaldehyde, the balance being the reaction

product with water, methylene diol. During storage, these solutions generate significant quantities of methanol and

formic acid. Despite this, there seems to be sufficient anecdotal evidence to support the use of formaldehyde with

copper. Short term exposures ( 1/2 to 1 hour) to about 100 ppm of this powerful irritant is tolerated by fish and seems

effective in forcing parasites to detach from fish. When used in a biologically filtered aquarium for an extended time

(days), 10-15 ppm have been recommended. This concentration, however, is deleterious to the filter bed and will

often result in 30-60% loss of nitrifying capacity. A comparison of the apparent synergistic action of formaldehyde with

copper indicates that 2-6 ppm formaldehyde is just as effective as 10-15 ppm and will effect the nitrifying capacity by

less than 5%. Such a low concentration of formaldehyde alone has little antibacterial, antifungal, or antiprotozoan

activity. What, then, is the mechanism of action for the combination of copper and formaldehyde? Up to now the only

copper mentioned has been cupric copper, the oxidized form of copper or copper with two positive charges. There is

another form of copper, the reduced form with only one positive charge, called cuprous copper. Cuprous salts are ten

times more toxic to life than cupric salts. The reduction of cupric salts to cuprous salts by aldehydes in alkaline

solutions is a well known reaction and is the probable mechanism for the increased effectiveness of copper in the

presence of formaldehyde. Under strongly alkaline conditions weakly complexed or chelated copper is reduced fairly

rapidly, while strongly complexed copper is not reduced. Close inspection of some commercial products containing

formaldehyde and copper under alkaline conditions will reveal a red, brown, or yellow precipitate, cuprous oxide.

Citrated copper products containing formaldehyde or another powerful reducing agent, vanadium, are usually acid

solutions to prevent this reaction in the bottle. Strongly complexed or chelated copper does not react this way in the

bottle. Since cuprous ions have not been detected in treated aquaria, and since formaldehyde does seem to increase

copper’s toxicity to parasites but not fish, informed conjecture suggests that, possibly, the reduction takes place after

copper has become membrane bound. One thing is certain, however, no red, brown, or yellow precipitate from a

copper product should be introduced into a marine aquarium. In marine water cuprous oxide will redissolve and,

although cuprous salts are reoxidized to cupric salts fairly rapidly, it can easily bring disaster in short order.

Some self-evident, but often ignored, principles in treating fish apply particularly to the use of copper:

(1) Unless absolutely necessary. do not treat in the display aquarium. Isolate the fish to one or more quarantine

tanks instead.

(2) lf treatment in a display tank is unavoidable, do not add anything to the water that cannot be removed. This

includes antibiotics, magic conditioners, drugs, dyes, and precipitating copper salts. Although these medications fall

out of solution, they have not been removed, but are only sitting in the filter bed, waiting for the worse possible time to

cause trouble. No amount of carbon or miracle filtration can remove precipitated chemicals. The only kinds of copper

that do not fall out of solution in the aquarium are chelated and amine-complexed copper. Amine-complexed copper

is removable with carbon: chelated copper is not removable except by water change.

(3)Avoid using anything that will interfere with biological filtration. If unavoidable, use only in a quarantine tank

without such a filter and make frequent water changes as indicated by ammonia levels. Chelated and aminecomplexed

copper do not interfere with biological filters. This is not always true of copper sulfate or citrate.

(4) Do not use anything ineffective or medication that will do more harm than good. A fish cured by death is hardly

a success. Chelated and amine-complexed copper are both relatively safer than copper salts. Amine-complexed

copper, however, is much more effective than chelated copper and is not as readily absorbed by fish.

(5) Always quarantine and treat new fish for prevalent infestations to minimize risk of introducing disease to

display tank. Always quarantine new invertebrates as well. Quarantine should be fora minimum of two weeks.

Never take a dealer’s word on the disease-free condition of the fish you just purchased. Unless he personally

quarantines and treats all his fish, he has no way of knowing that a fish is free of disease.

FIGURE 1

Figure 1. The dissolving of copper chloride in water. The symbols are Cu = copper; Cl chloride; 0 = oxygen; H =

salt like copper chloride is added to it, water molecules, which are present in excess, arrange their negative charges

around the positively charged cupric ion and their positive charges around the negatively charged chloride ions,

effectively separating the components of the salt into dissociated cupric and chloride ions. The long arrow indicates

that this dissociation takes place much more extensively than the reassociation of the ions (short arrow).

Figure 2.

The formation of chloride-copper complex in marine water. The symbols are the same as in Figure 1. Observe that

water forms a four-membered complex with copper, leaving the cupric positive charges intact. In the presence of

excess chloride ions, as in the marine aquarium, chloride displaces water and forms a four-membered

chloride.copper complex that has two negative charges. The arrows indicate that the reaction is reversible. but the

length of the arrows show that the chloride-copper complex is the predominant form. This is true of any ionizing

copper salt added to marine water.

FIGURE 3

Figure 3. Representative copper complexes and chelates. The symbols are the same as in Figure 1 with the addition

and that of an ammonia analog or amine where some of the hydrogens of ammonia are replaced by any of several

thousand possible R groups. One such R group yields Tris, an often used fish buffer. The rest of the complexes

shown involve ring structures and are, consequently, chelates. Cupric acetate is a very unstable structure which may

not actually ever exist as shown; but acetate does form a weak complex with copper. Glycine is an amino acid and

occurs naturally in the aquarium; it is a good chelating agent. EDTA (ethvlenediamine tetraacetate) is a very powerful

sequestering agent that forms triple locked rings with copper; it, or closely related sequestering agents, are used in

commercial "chelated copper" products. Citrate is an unstable weakly chelating salt used to maintain stock solutions

of copper. Observe that ammonia and amine complexes retain the positive cupric charge, while the other complexes

are either neutral or negatively charged. Note also that all the stable complexes are linked to copper through

nitrogen..