Most saltwater fish we see in the hobby today are still wild-caught. Wild-caught fish, fresh and salt water, often carry external parasites. In the wild, these external parasites rarely, if ever, become a real problem because the lifecycle of these parasites prevents them from reaching plague proportions in the open ocean.
When we bring wild fish into our closed systems with a small amount of water compared to the ocean, we increase the chance that these parasites can find a host and multiply out of control. By far, the number one cause of a total aquarium fish population being wiped out for marine hobbyists is an external parasite population explosion.
The most common marine external parasite that can be a problem for marine fish in captivity is marine ich or velvet disease (Amyloodinium ocellatum and Cryptocaryon irritans), and black ich (Paravortex). All of these parasites have similar lifecycles. Once the parasite matures, it falls off the fish, breaking open and releasing hundreds of new parasites into the water column. These parasites have limited time to find a host (your fish) before they die.
In the open ocean, these parasites have a couple of factors working against them, the population density of fish is very light as compared to the aquarium, so they have a hard time finding a host before they die, and many filter-feeding invertebrates, corals, and worms will feed on them. In the aquarium, we have only a tiny amount of water, which makes it easy for these parasites to find a host. Within weeks after introduction, these parasites can reach plague proportions and wipe out an entire fish tank.
Marine ich, Amyloodinium, and Cryptocaryon appear on the fish as tiny white dots about the size of a period (.). While you see the parasites on the fish's body and fins, it also invades the gills, making it very difficult for the fish to get oxygen to the blood. When fish are heavily infested, they die of hypoxia (suffocation).
Black ich, Paravortex is often found on yellow tangs (Zebrasoma flavescens) and appears on the fish as black spots the size of a period (.). These spots are turbellarian flatworms. This parasite can kill fish in the same way as Amyloodinium and Cryptocaryon.
To prevent these parasites from invading your display tank, quarantine all new fish for at least three weeks in a separate tank. The quarantine tank needs to be set up for use with copper medication. I usually use about a 20-gallon (80 l) tank for this process, but the actual size you use can vary depending on the size of your new fish. I use a hang-on-the-back power filter with no carbon to set up this tank. Carbon will remove medication, so it has no place in a quarantine tank. A quarantine tank also needs a heater, as you must keep the water warm, around 85° F (29.5° C). The tank should be kept bare bottom (no sand or gravel) as calcium-based substrate can pull copper medication out of the solution.
It is essential to have beneficial bacteria established in the quarantine to keep ammonia and nitrite from accumulating. You can introduce beneficial bacteria to the quarantine tank by adding filter media or decorations from an established aquarium and by bacteria in a bottle product with marine nitrifiers.
I prefer to use a chelated copper like Copper Power. Chelated copper is more stable than non-chelated, staying in solution for much longer. Non-chelated copper quickly falls out of the solution and often requires frequent dosing. Chelated copper is also safer for fish and is used at a higher concentration than non-chelated copper. Copper can be a deadly medication for fish if overdosed, so you must not exceed the manufacturer's recommended copper concentration.
Before you add a second dose of copper to the tank, you must check the residual copper concentration level in the tank with a copper test kit that can detect the type of copper you use. Not all copper test kits can detect chelated and/or non-chelated copper, so it is best to check with the manufacturer before you buy. Once you have tested the copper level, add only a percentage of the full dose required to increase the concentration to the effective level. In my experience, copper at the concentrations used to treat fish for external parasites does not adversely affect ammonia and nitrite-reducing bacteria.
While on the fish, the parasites burrow under the fish's skin and have this protective layer that helps shield them from the copper in the water. When the parasites drop off the fish to multiply, they are then vulnerable to the killing power of copper. You can speed up the parasite's lifecycle by keeping the aquarium water warm (85° F/29.5° C).
If you notice parasites on the fish, you can give the fish a freshwater bath for 3 to 9 minutes. This will often kill many parasites on the fish, but it cannot be used as an absolute way of making fish parasite free. Often some parasites will survive a freshwater bath. To prepare a freshwater bath, fill a small container with tap water at the same temperature as the aquarium. Dechlorinate the water with any commercially available dechlorinator aquarium product. Net the fish out and add the fish to the container of freshwater. Many species of fish will act abnormally in freshwater, which is normal. Monitor the fish closely to make sure the fish is still breathing. If the fish looks overly stressed, you can return the fish to the quarantine tank. Freshwater bath treatment can be repeated every day if necessary.
Sometimes newly acquired fish (especially butterflyfish and angelfish) will come down with a bacterial infection from the stress of transportation within a couple of weeks of being purchased. If you notice a bacterial infection, see Aquaworld Aquarium's article Treating Marine Fish for Bacterial Diseases. Bacterial diseases can be treated at the same time you treat them with copper, but I only recommend treating them with an antibiotic when necessary.
Some fish, like sharks and rays, are reportedly very copper-sensitive. I have not tried treating them with copper, so I can not confirm if this treatment would be safe for these types of fish. But with all other common marine fish kept in the hobby, I have not seen any adverse effects of long-term exposure (one year plus) to chelated copper at levels used to kill external parasites.
Quarantining all new marine fish in a tank with copper should be a standard routine for all marine hobbyists. Setting up a quarantine tank will save the hobbyist money in the long run and prevent many premature deaths of marine fish in captivity. Treating a reef tank for a parasite outbreak is difficult and often unsuccessful. The best practice is prevention.