Provided is a purification method for water to be used for culture of an aquatic organism, wherein the method includes a nitrification step of oxidizing ammonia to nitric acid using a nitrifying bacterium adhering to a base material containing an alkaline earth metal, and a denitrification step of reducing nitric acid to nitrogen using a denitrifying bacterium adhering to a base material containing a biodegradable resin which has a structural unit derived from dicarboxylic acid.
