Quantine, or whatever that Q word is.
UV does not prevent disease. It only kill certain transmitable disease when the pathogen pass by the lamp and get zapped. Some of the pathogen may get from one host to another within the tank without getting pump thru the UV.
It is most useful in certain setup, for example, when all the input water must go thru the UV, such as the case of open system setup where the seawater is UV'ed before introduced into the display tank. Most major aquarium located right next to the ocean use this method (Scrippt in CA and Wakiki in HI, I think). Another example is a connected tank setup similar to tank-sump system where all the water is UV before pump to the other tank. This way, you can have a tank with sick fishes without worry about affection another connected tank with health fishes. But once a formerly "healthy" tank got problem (stress, parameter shock, etc.) then the illness with spread among the fishes within the same tank.
