The invention includes a composition for stabilizing chlorinated water to sunlight decomposition, and methods of preparing compositions. The composition is a slurry composition of a monoalkali metal cyanurate, of low viscosity. Two methods of preparing the slurry are described, in which cyanuric acid or cyanuric acid wetcake is mixed with a monoalkali metal base. One method dry blends cyanuric acid or cyanuric acid wetcake with a monoalkali metal base. The invention also describes a method of preparing a dry, solid monoalkali metal cyanurate.