At some point in your journey as an aquarist, odds are great you’ll experience this parasite. It won’t be pleasant, and you’ll likely lose at least one fish (if not more). Called ick, ich, or white spot disease, it’s a common pathogen and it’s deadly, but it’s not difficult to treat.
I’ve been there. Fortunately, I received great advice on how to treat it and I succeeded without losing any fish. My outbreak resulted from new fish that I did not quarantine.
So how do you prevent it and how do you treat it? Most experienced aquarists won’t find anything new on this post, but if you aren’t familiar with ick, see this article from Tropical Fish Hobbyist a few years ago.
UPDATE 1/23/20: More detailed information about ich, including prevention and treatment, can be found in this article from The Witty Fish website. Shout out to Tim Kerrigan, the site’s proprietor.