A composition of alkali carbonate peroxyhydrate in a quantity of 45 to 85% by weight relative to a total amount of the composition and an alkali hydrogen carbonate is provided for combating pathogenic germs and parasites in water of aquaculture, ponds, and aquariums； for destroying thread algae in water； for processing water. The addition of alkali hydrogen carbonate to alkali percarbonate peroxyhydrate stabilizes the alkali percarbonate peroxyhydrates relative to combustible materials. The alkali carbonate peroxyhydrate is preferably Na2CO3 and xH2O2.
