A method for inhibiting or preventing emergence and growth of algae, comprising use of a germanium compound and a herbicide, and an antialgal composition comprising a germanium compound and a herbicide. The method of the present invention inhibits and prevents emergence and growth of algae, and can be used safely with little influence on aquatic animals. The method is advantageously used for a fish tank in which fish for appreciation is kept, so as to keep good appearance of the tank and to reduce burden of cleaning the tank by preventing emergence and growth of attached algae. Particularly, the composition for artificial seawater of the present invention shows no toxicity to marine animals to be raised in the artificial seawater prepared using the composition of the invention, and can inhibit and prevent emergence of algae. Therefore, the composition of the invention can be beneficially used for raising, culture, study, and the like of marine animals. Moreover, the composition of the present invention which is coated on a fish net, a hull, or the like can result in inhibition and prevention of emergence and growth of algae in the environment where influence on fish, etc. is very little.