A method of cleaning water systems and an oxidizer (e.g., a potassium monopersulfate composition) that is used for the method are presented. When potassium monopersulfate is used as the oxidizer, it preferably has a low concentration (<0.5 wt. %) of potassium oxodisulfate byproduct that causes irritation. The low potassium oxodisulfate concentration allows the composition to be used more liberally than conventional potassium monopersulfate. To control the release rate of the oxidizer, the oxidizer is formed into a tablet and placed in an enclosure. The enclosure has pores such that water enters the enclosure at a controlled rate and dissolves the oxidizer. The oxidizer solution that is generated in the enclosure enters the water system through the pores. Optionally, the composition may also include a layer of coating material either under or over the enclosure that further controls the rate of oxidizer dissolution.