Interesting read if you haven't already:
http://www.reefkeeping.com/issues/2003-08/sp/index.php
"Copper is a highly effective medication against Cryptocaryon irritans when dosed and maintained in the proper concentration. The references I found varied in their recommended dosage:
Andrews et al, 1988: 0.15-0.30 mg/l
Bassleer, 1996: 0.25-0.30 mg/l
Gratzek et al, 1992: 0.115-0.18 mg/l
Noga, 2000: 0.15-0.20 mg/l
Untergasser, 1989: 0.15-0.20 mg/l*
*(recommends to be used with Methylene Blue)
I am going to abbreviate my advice and simply suggest to: "Always follow the directions of the manufacturer of whichever brand of copper medication you employ, and always use a test kit to verify the dosages." Copper has a narrow range of effectiveness and levels must be monitored at least daily.
Copper has several disadvantages in treating Ich. First, at too low a dosage, it is ineffective. Secondly, at too high a dosage, it could kill all your fish. Daily, or better yet twice daily, testing is required to maintain an appropriate and consistent level of copper. Even when within the appropriate ranges, some fish cannot tolerate copper. Some of the fish more sensitive to copper are lionfish, pufferfish, mandarins, blennies, and any other scaleless fish. Copper is also a known immunosuppressive, making fish more susceptible to secondary infections. Invertebrates are extremely sensitive to copper and cannot be housed in a tank undergoing this treatment. Lastly, copper cannot be used in the presence of any calcareous media. Live rock, sand, crushed coral, and dead coral skeletons will all adsorb copper, rendering it useless a treatment.
Copper specifically targets the infectious, free-swimming theront stage of this disease, as being buried deep in the skin of the host protects the trophonts; the cyst walls of the tomonts are similarly impervious (Colorni & Burgess, 1997). Knowing this and the life cycle of Cryptocaryon irritans, monitoring and dosing as needed in the evening right before the lights go out is going to be the most effective method. This should ensure optimal treatment concentrations at the most beneficial time.
Copper is probably the most popular method of treating Cryptocaryon irritans, but is not my first choice. It is far too labor intensive for me to recommend to the general public, has a large risk of overdose, lowers the fish's resistance to other diseases, and can cause serious damage to the kidney, liver, and beneficial intestinal flora of the fish being treated. Damage to intestinal flora is what many hobbyists point to as a possible contributing cause for Head and Lateral Line Erosion (HLLE), although there is currently no definitive cause of HLLE."
