Ichthyophthirius multifiliis is the causative agent of white spot disease, commonly known as "Ich".  Its marine counterpart is Cryptokaryon irritans. 

Life Cycle:  Understanding the life cycle of the Ich organism is important in understanding appropriate treatment.

Life cycle of Ich, courtesy of Univ. of Florida, IFAS

White spots are visible on the fish due to the encysted trophont feeding stage.  The trophont eventually enlarges, breaks through the skin and attaches to substrate and is referred to as the tomont.  These tomonts undergo mitosis forming tomites which are released as hundreds of free-livingree living threonts.  Threonts penetrate the skin and gill epithelium and enlarge forming trophonts.  Diagnosis is made by microscopic examination of skin or gills revealing trophonts with a horseshoe shaped nucleus.  The only stage that is susceptible to treatment is the free-living threont stage!  This free living stage lives 24-48 hours and will die if it does not find a fish host.  Therefore multiple treatments are necessary to eliminate the parasite. Common treatments include salt,  formalin and malachite green. 

Treatment is temperature dependent.  The higher the temperature the quicker the life cycle.