I had always been told that Malachite Green contained Copper and that's what made it Green. I believed it until someone told me that the dye was only named after, and not made from, the mineral. It's just one of the several myths in fishkeeping that is often repeated and reprinted, like you need airstones to oxygenate a tank and you need to buy bacteria to keep your biofilter healthy.
Copper is sometimes used for treating aquaria parasites because it's toxic at pretty low levels to parasites (and Plants)! At higher levels it severely stresses fish and at yet higher doses, its lethal. Mardel makes Copper products for killing ick -- one is called Coppersafe but I think that is an oxymoron. I don't know anyone else that makes it for that purpose but there might be some out there. From what I have seen, copper has fallen into disuse as anti-parasitic because Malachite Green and formalin are very effective at nontoxic levels to plants and less stressful levels to fish.
Twenty-five years ago (as today) Copper was sold to treat ick, but back then people were also still selling Methylene Blue under the mistaken belief that it had antibiotic or anti-parasitic properties rather than merely retarding some bacterial growth. The aniline dye known as Malachite Green (also known as benzaldehyde Green) is a particular chemical that is made up of carbon, hydrogen, nitrogen, and chlorine atoms. I don't think a manufacturer could have legally labeled a product as "Malachite Green" without labeling the container as containing Malachite Green (a particular chemical) AND Copper if it contained Copper. . .But who knows. . .Twenty-five years ago the vendors told us we needed air pumps and air stones to keep the water oxygenated.
Formalin (solution of CH2O, formaldehyde, in water) is also used as fish
medicine in place of or mixed with Malachite Green. For example, Quick Cure
is such a mixture. Although formalin is extremely effective, it is reportedly
more toxic than Malachite Green. It's extremely toxic at high
concentrations. It's the chief constituent of embalming fluid.