ich is caused by ich being present in the water, usually a newly added individual becomes susceptible because of stress and a lowered immune capacity due to stress. older tank mates may get it due to stress from the new individual being introduced messing with the balance that has been reached so far.
What causes marine "Ich"?
Marine "Ich" is caused by a ciliated protozoan called Cryptocaryon irritans Brown 1951. Ciliates (Phylum Ciliophora) are one of the largest groups of protozoans and all posses cilia or compound ciliary structures for food acquisition or locomotion at some point in their life cycle. Of the some 7200 species that have been described, around one third are ecto- and endo- commensals and parasites (Ruppert and Barnes, 1994). C. irritans is an obligate ectoparasite (Dickerson and Clark, 1996) which means that it is an external parasite that needs the fish host to complete its life cycle. The name 'Marine "Ich"' has been coined because it is essentially the marine equivalent on Ichthyophthirius multifiliis Fouquet, 1876, or "Ich" for short. While the two species of ciliates have superficially similar life cycles and signs, they have been found to be only distantly related (Wright and Colorni, 2002) .
The life cycle stages of C. irritans
C. irritans has a four stage life cycle. The parasitic stage (trophonts) is the one that results in the appearance of white spots all over the fish. The trophonts burrow under the skin where they feed on body fluids and tissue debris. When the trophonts first infect the fish they are small but grow as they feed and so the white spots are initially small but get larger as they mature. Once mature, they drop off the fish and sink/swim down to the substrate where they encyst and begin to reproduce. In this stage they are called tomonts. After a number of days in which the tomonts divide, the cyst ruptures, releasing the tomites. Tomites may differentiate into theronts, the infective stage, which actively seek a host to reinfect.