Abstract: Means are disclosed for introducing a biological poison to water being removed from a lake or stream for presentation to an automatically operating water quality monitoring system. The poison introducing means is operative at only periodic intervals; and, when operating to introduce poison, a sampling device of the monitoring system is disabled to preserve its accuracy. The monitoring system is otherwise permitted to operate routinely during the time the poison is present in the system. The poison is admixed to the water promptly as the water is introduced to the monitoring system, with the result that the biological poison will cleanse the entire monitoring system by killing biological slime otherwise adhering to the exposed surfaces within the system. The biological poison also maintains sensor sensitivity within the monitoring system.