The dreaded Ich (Ichthyophthirius multifiliis) has struck my community tank. And in the last day or so I have learned an awful lot about the disease.
Ich is a disease caused by a parasitic ciliate. I got the impression initially that it was a disease of low water quality - that the organism was usually present in aquaria, but only stressed fish suffered actual outbreaks. Turns out that bit of conventional wisdom is false - it's an obligate parasite, so any introduction needs to come either with living fish or live plants. I hadn't thought of quarantining plants (well, slightly, but not to any great extent).
So how did I get it? The outbreak seems to be confined to the platies, but it's almost impossible that they are the cause. Most of my platies were born in this tank - they were the first fish I got. So the true culprits must either be plants (which are, after all, my most recent purchases) or an asymptomatic fish ("asymptomatic" in the sense that I can't see the symptoms, although they could easily be on the gills). I've had my newest fish (the Otocinclus and the Kuhli loaches) for two and a half weeks, so they could be the culprits, as could the new plants.