Free copper Cu is toxic to marine life including invertebrates and even fish at levels higher than therapuetic levels,. Fortunately, most of the copper that comes into our tanks with foods or salt mixes is bound to organics or quickly becomes so and as such doesn't pass through cell walls easily if at all. Very small amounts of free copper in parts per billion can wipe out corals and other invertebrates quickly.
Free copper will adhere to live rock , substrate and other calcium carbonate surfaces making the maintenance of a therapuetic level diffiucult without frequent testing. Further these materials will hold the copper and leach it back overtime ending their suitability for use in a reef tank unless they are partially dissolved with an acid bath.
Copper meds available today come in different forms. Some are free Cu+6
straight up, such as copper sulfate meds . These require careful testing to maintian a therapuetic level. There are others like Cupramine which provide copper that is bound to orgaincs(probably amines) which dissolve to certain levels of equilibirum ,ie, releasing free copper at a rate to maintain a therapuetic dose consistently.
Some fish, are beleived to be particularly sensitive to copper treatment. Chronic exposure to copper can cause damage to internal organs which is another argument against not maintaining tanks with copper for long periods of time.
Nevertheless, a 14 day treatment at therapuetic levels has been very effective against ich and velvet , ime, with no discernible ill effects to the variety of fish I've treated this way. I use a treatment tank with some pvc for resting places and pre seeded sponge filters for nitrification. Fro wrasse, I add a container of sand for them to sleep in.
Hyposalinity is a usually viable alternative for ich treatment but it takes much longer often 6 weeks or more and in some cases is not effective depending on the strain of ich. Prolonged exposure means the fish don't use their kidneys or flush them for long periods of time which can be harmful.
It works on the weakness of the free swimming stage of the parasite in that the theront is isotonic(it's internal sg matches the water around it) It can't osmoregulate like a fish can. It absorbs fluids that diffuse into it from the the lower sg water and can't maintain homeostasis. The same is true for all invertebrates. Fish have an internal salinity, just as we do. Marine fish are hypotonic;their internal sg is around 1.008 ,much lower than the surrounding water in which they live, typically 1.026. They maintain the 1.008 internally by copious drinking and expelling concentrated urine. They can not maitain homestasis if the surounding water is lower than their internal sg. This is why 1.009 is the lower limit for hypo treatments and it may not be lethal to all strains of ich or cysts in a reasonable time frame.
A very nice way to treat ich is tank transfer. This requires transferring the fish every 3 days to a uninfected tank for atotal of 4 transfers. this will ensure all the parasites in the fish (trpophonts) will be left in the vacant tank and no new ones will have time to hatch. Nice and natural but labor intensive.Each tank and equipment used in it needs to be completely dried before reuse or soaked in fresh water overnight. Still it's hard to debate the elegance and natural efficacy of this technique.
The ich itself creates lots of open wounds and weakens the fish so secondary bacterial and/or dinoflagellate infections have plenty off ferile ground even if they don't cut themselves.
