Because marine ich is difficult to treat without harming reef aquarium invertebrates, prevention is definitely better than cure.
Salt grain-sized white cysts are visible on the skin and fins; cysts are also present on the gills. Fish may scratch against solid objects, and breathing may be labored. Excess mucous may be visible as gray patches on the body.
Marine velvet is similar, but the cysts are smaller, more like powdered sugar than salt. Velvet makes a fish look like it has been dusted with a fine powder, whereas ich is more like salt grains. Velvet is almost always associated with very rapid ventilation of the gills.
The causative agent is the protozoan parasite Cryptocaryon irritans.
There are four stages in the marine ich life cycle.
The feeding stage is known as trophont (or trophozoite). As the trophont feeds on tissue fluids, the distinctive white spot forms around the parasite as a cyst.
Once the trophont is mature, the cyst bursts and the free-living protomont emerges. Secondary infections can set in at this stage because of the damage caused to the host fish’s skin. Within a few hours the protomont settles onto a solid object, potentially even wet objects, such as nets, that can be moved between aquariums.
Protomonts form capsules and begin dividing, eventually forming hundreds of infective cells known as theronts or tomites. In a tropical marine aquarium, this reproductive stage lasts for about three days.
After the reproductive phase is finished, the capsule bursts, and the theronts swim into the water and seek out new hosts. The theronts will die after a few days if they cannot find a suitable host.
The marine ich parasite is only vulnerable to treatment during the free-living stages of its life cycle. The non-free-living trophonts are too well-protected by the skin and mucous of the host fish to be treated.
When treating ich, the temperature of the aquarium is often raised a few degrees above normal. This speeds up the life cycle of the parasite to its vulnerable free-living stages more quickly. However, solubility of oxygen in seawater goes down as temperature goes up, so additional circulation and aeration may be required.
Medications containing copper (typically copper sulfate) are very effective at killing off the free-living marine ich parasites. A test kit must be used to ensure the copper concentration stays within safe levels. These medications can be very useful in fish-only aquariums, hospital aquariums, quarantine aquariums and as baths for dipping new livestock.
Copper is acutely toxic to most marine organisms. While bony fish will generally tolerate therapeutic concentrations of copper, cartilaginous fish, corals, invertebrates, living rock and macroalgae will all be harmed or killed by copper. Obviously, copper-based medications cannot be used in FOWLR (fish only with live rock) or reef aquariums.
Another way to treat ich is to gradually reduce the salinity of the aquarium by about 2 ppt (parts per thousand) per day until the salinity reaches 18 ppt (about half the normal salinity of seawater). Maintaining the salinity at this reduced level for a month or so will kill off any free-living parasites.
Again, this is normally only an option in aquariums containing bony fish. Corals, living rock, macroalgae, and most cartilaginous fish and invertebrates will not adapt to reduced salinities.
Monitoring and controlling water chemistry is very important. Use chemical buffers to ensure pH and alkalinity remain constant, despite the reduced salinity. Also note that temperature affects specific gravity, not just salinity. So, while a salinity of 18 ppt corresponds to a specific gravity of 1.012 at 77 degrees Fahrenheit, the specific gravity will be closer to 1.010 at 86 degrees.
Some strains of Cryptocaryon irritans are more tolerant of low salinities than others, in which case reducing the salinity further and/or extending the period of hyposalinity may be required.
Treating Reef Aquariums
Because copper and reduced salinity regimes cannot be used to treat reef aquariums, the only completely safe way to deal with marine ich in a reef aquarium is to remove all of your saltwater fish to a hospital aquarium where they can be treated separately.
Once the reef aquarium is devoid of saltwater fish, the parasites will not be able to find hosts and will eventually die. This takes a minimum of four weeks.
Because treating marine ich is not easy in aquariums containing anything other than bony fish, prevention is the best way to manage this particular disease. All new livestock should be quarantined for a minimum of four weeks prior to introduction. This will provide ample time to detect, and if necessary treat, ich infections.
Ultraviolet sterilization has sometimes been touted as a useful way to deal with marine ich. While UV light can kill the free-living parasites, the efficacy of a UV sterilizer depends upon proper cleaning and maintenance, and in practice UV sterilization should be viewed only as a supplement to proper quarantining and treatment, not an alternative.