Marine ich is one of the most commonly occurring diseases in saltwater aquariums. It is caused by a parasite named Cryptocaryon irritans. It can infect almost all marine fish species but some fish are more likely to get this disease than others. Surgeonfish are among the fish that are most likely to be infected by marine ich. This disease does not infect sharks and rays.

The parasite that causes marine ich has a life cycle very similar to that of freshwater ich. It spends a part of its life attached to a fish feeding of it. It then falls off and attaches itself to a surface and forms a cyst where it divides itself into hundreds of new parasites. It can stay in this state for almost a month until the new parasites are released into the water and start looking for a host. Once the parasite finds a suitable host, the cycle starts over. Their life cycle can sometimes give the illusion that the disease has been eradicated when in fact it is only in its dormant stage. This is why it is very important never to stop treatment prematurely.

The symptoms of marine ich are much the same as those occurring in fish infected with freshwater ich and include white spots on the body and scratching. To learn more about the symptoms I recommend reading one of our articles about freshwater ich.
Preventing marine ich

The best way to treat a disease is always to prevent it all together. To do this you should always keep good water quality in your aquariums and your fish in good condition. All new fish should ideally be quarantined for at least a month. This gives any disease time to develop and the fish time to recuperate from the stress and potential damage caused by the transport as well. This also gives you time to feed the fish and improve its general condition. If the fish tolerates it, it is a good idea to keep a high temperature in the quarantine tank to speed up the life cycle of parasites and increase the chance of them being discovered. Using an ozone filter can also greatly reduce the risk for marine ich and even help eliminate the disease in infected tanks.

Treating marine ich

There are many ways to treat marine ich and I will describe some but not all of them below.  One possible treatment is to add copper to the water (0.15-0.25 mg/l or according to instructions). Copper is a very effective treatment but it should never be used in aquariums with lower life forms such as snails since it can kill them. Too much copper will kill your fish as well. Some fish species such as puffers are more sensitive towards copper than others. It is therefore important to research all the species in your aquarium before you use copper. Copper also has the disadvantage that it can't be used in an aquarium with live rock and other calcareous media (coral sands etc) as they absorb the copper. Copper is one of the most commonly used treatments but not one of the best.

Another treatment option for marine ich is to decrease the salinity of the water (1.009-1.010). A low salinity environment will kill the parasite and is harmless for most fish. You should keep the low salinity for 2-3 weeks. If you use this method it is important not to raise the salinity too quickly once the treatment is done. This method is not suitable if you keep invertebrates in your tank and some fish species will not tolerate it.

The least intrusive way to treat this disease is by changing 50% of the water each day for two weeks. This removes the parasites. This method is most suited to be used for treating one fish in a hospital tank and not for an entire aquarium. It is very important to match the temperature and salinity of the old water when using this method.

Malachite green can be used to treat this disease but is often less effective than above mentioned methods. Malachite green is supposedly reef safe (I haven't tried it myself) but I still recommend treating in a hospital tank if possible.

Mild cases of marine ich can be treated biologically with the help of cleaner shrimp. Cleaner shrimp can also help prevent the disease from ever erupting. This method is however seldom very effective if the disease has become more widespread and if the disease seems to progress despite the presence of cleaner shrimp one should use other treatments.

Medicated food can be very good for treating marine ich in show aquariums or when you for other reasons can't move the fish to treat them in hospital tanks. Mix Metronidazole into the food and make sure that the infected fish eats enough of it.  Medicated food combined with cleaner shrimp can often be a very good way of treating marine ich in your show aquarium.

There are reports that garlic, ginger and pepper can be used to cure this disease. I will not recommend using any of them until proof of their effectiveness have been presented.