Most aquarium keepers at one time or another have probably battled ich on one of their tanks at least once. Officially called Ichthyophthirius, it is better known as White Spot or ich.
                     
What is Ich? The life cycle of the Ichthyophthirius parasite is a complex one. The white spot, known as a trophont, forms under the skin or the gill epithelium, and once there is burrows into the skin of the fish. Once in the skin it constantly wiggles and moves killing the surrounding cells. The trophont will then feed on the dead cells and the body fluids of it’s host. Once it has reached maturity, the parasite will extricate itself from it’s host and seek out an object in the tank to which it attaches itself. Once it attaches to that surface it forms a “cocoon” around its body. Once inside the cocoon the parasite is know known as a tomont. Inside the cocoon the tomont begins to multiply exponentially by a process of dividing itself; this process can result in the production of nearly one thousand new parasites. These new parasites are known as tomites. When the tomites emerge from the cocoon they are free swimming and they seek out a new host and the process begins again. The entire life cycle of Ich is greatly dependent on the temperature of its environment. In water that measures 45 degrees Fahrenheit (7 C), the life cycle will last up to six weeks, but in water that is 77 degrees (25 C) the cycle is only one week.
                     
What are the dangers of Ich? As has already been discussed, the ich parasite creates tissue damage in the host fish. This tissue damage can become the site of a secondary bacterial or even a fungal infection. In large numbers, the ich can create a significant amount of tissue damage to the fish, which can result in its death.
                     
How do I identify Ich? One of the earliest sings of ich can be that a fish starts to display the “flashing” behavior, unfortunately this not a sure sign of ich since fish will also do this when the water quality is poor. The first sure sign that a fish has ich will be the white cysts that form on the body and fins of the fish. The fish behavior will likely change first by the fish scraping itself on just about anything it can find. In the late stages of the disease the fish will likely become quite lethargic and simply sit on the bottom most of the time.
                     
Treatment. Treatment of ich is generally not as difficult as it may at first appear. The parasite when under the epithelium, in the white cyst, and in the cocoon is completely immune to treatment and can only be treated while free swimming. There are a variety of medications available for the treatment of ich; Quick Cure and RidIch are just a couple of them. Both of these medications contain the same ingredients: malachite green and formaldehyde. Both of these meds can have a devastating effect on the parasite but unfortunately they can also have that same effect on your biofilter. However if you do choose to use either of these meds take note of the instructions on the bottle. According to Doug Thamm of School of Veterinary Medicine of University of Pennsylvania, the directions on those bottles should be ignored. The instructions will tell you to treat only once or twice, but this will not rid your tank of the parasite, it will simply get it under control for a short time and you will be using it again shortly. Treatment must be continued for 12-16 days.
                     
There is an inexpensive alternative to the pricey medications available to the aquarium keeper. Salt, at high enough concentrations, is lethal to the ich parasite; this is especially true during the tomite stage of its life cycle. To treat ich in this way the aquarist should first raise the temperature in the tank to roughly 80 degrees Fahrenheit to speed the life cycle. At the same time a one third water change should also be performed. When replacing the water in the tank the salt should be dissolved in the new water before adding it. Depending on the fish you have, a concentration of 1 tablespoon per 5 gallons is the desired concentration of salt. If, however, you have fish that are sensitive to salt (i.e. Clown Loaches, and other scaleless fish) in the water this concentration should be lowered to 1 tablespoon per 10 gallons of water. Which ever method of treatment you use, medication or salt, Melefix should also be used in conjunction. As has been noted earlier, the ich creates areas of destroyed tissue where a bacterial or fungal infection may occur. To prevent these secondary infections Melefix should added to the water as per the instructions. Furthermore, during the entire treatment process, do not neglect your regular water changes in order to keep up the water quality.
                     
Just a quick note on the newest strain of Ich. Often known as “Super-Ich,” this new strain is resistant to salt and heat treatment. The only medication that this effective with this new strain is Quinine based. Copper based treatments and all other such medications are completely useless against this new strain. It is likely that this new very resistant strain of Ich developed as a result of insufficient doses of medications were administered. The strength of medications is carefully calculated so that it will kill the parasites, when you lower the concentration of the med you give the parasite the chance to adapt to it and become completely resistant to it. The speculation is that this is exactly what happened. People treated with “half doses” because of more sensitive fish. The fish can handle the full doses.
                     
How did Ich get in the tank? Ich has often been attributed to poor water quality, and this is true. However, water quality alone is not the only cause, since it is a parasite it had to come from somewhere. Ich often enters the aquarium on plants, driftwood, or even newly added fish. Poor water quality, however, does create an environment that is much more conducive to the growth of ich. For this reason it is important to keep your water quality high regardless of what fish you have.
                     
A marine form of the disease also exists however treatment is different. For obvious reasons the salt method does not work. Many of the medications that exist for treatment of marine ich are copper based and thus are unable to be used in a system where invertebrates are present. If nor inverts are present then these meds can be added directly to the tank. If inverts are present then a non-copper based product must be used for the tank.
                     
Ich prevention Prevention is fairly simple. Keep you tank clean with regular water changes and gravel vacuuming, and examine new fish carefully. To prevent Ich from getting into your large system a smaller quarantine tank should be established and new fish should be held there for 2 weeks to allow any possible problems to come out.

Sources:
http://www.fishdoc.co.uk/disease/whitespot.htm
http://www.duboisi.com/diseases/ich2.html