There is no big difference between Ick and Ich, as both are referring to the same thing. Even aside from this fact, both words are pronounced the same way. Ick and Ich are both short forms of a parasitic infection of freshwater fishes known as Ichthyophthiriasis. This disease is caused by an organism known as Ichthyophthirius multi-files.
This disease is one of the main causes of death among fishes, especially among those in water tanks or aquariums. However, another name for this disease is white spot disease. Freshwater fishes are in their most vulnerable state when they have a low immune system in their bodies.
Meanwhile, one of the reasons why most fishes have a low immune system is due to stress. When a fish is infected, one of the things that will show that a fish has been infected is that there will be the appearance of a white spot all over the scales and gills of an infected fish.