Despite a long history of the use of copper, the preferred agent for the eradication of external parasites from marine fish, many discordant recommendations of expert aquarists betray a limited understanding of the basic chemistry of copper in the marine aquarium

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 See Fig. 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? 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 water-copper complex is so stable that when a solution of copper chloride is evaporated the complex does not break down and the final substance is actually [Cu(H₂O)₂]^{++ --}Cl₂ . In the presence of excess chloride ions, as in the marine aquarium, the predominant form is [CuCl₄]^--, a relatively stable complex ion that forms regardless of whether cupric 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 typical nitrogen complex of copper is ammoniated copper, [Cu(NH₃)₄]⁺⁺ ( Figure 3). Although ammoniated copper 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 use of any ionizing copper salt is equivalent and will yield [CuCl₄]^--, a negatively charged complex that is only 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.