There are provided entrapping immobilization pellets (14) which can exhibit purification performance immediately after being placed in an aquarium (12) and furthermore can provide a color, luster, and the like for the main body or periphery of the aquarium (12) to improve the beauty of the whole aquarium environment. There are provided entrapping immobilization pellets (14) for purifying breeding water in an aquarium (12) to breed aquatic animals (20), wherein microorganisms are entrapped and immobilized in an aqueous polymer gel and the entrapping immobilization pellets (14) are colored.