It's ich right? The name comes from the freshwater version of ich, which is caused by the parasite Ichthyophthirius multifiliis. The shortened name is obviously derived from the first few letters of the scientific name. The marine version is caused by a totally different parasite with a different name (Cryptocaryon irritans), but it still makes sense to call it ich.
Disclaimer: I know the spelling of the name is irrelevant, but the fact that it's inconsistent annoys my OCD.
Disclaimer: I know the spelling of the name is irrelevant, but the fact that it's inconsistent annoys my OCD.