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 amine-complexed 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.