PURPOSE: To make it possible visualize a watery scene in which fish swims in a natural environment with algae by arranging an ejector, a nitrification module, a sterilization/algal extermination module with an ultraviolet beam lamp and a denitrification module with photosynthetic bacteria in the forced circulation flow passage of an aquarium.

CONSTITUTION: Water is forcibly circulated using a pump 2 in an aquarium 1, and water containing air enters a nitrification module 5 filled with mineral 4 using an ejector 3. As this mineral 4 has nitrifying bacteria attaching to the surface, so that ammonia secreted by fish is oxidized into nitric acid. The water thus rendered harmless is sterilized with the extermination of algae using a sterilization/algal extermination module 6 equipped with an ultraviolet beam lamp 7 capable of generating an ultraviolet dosage of more than 4.95×104W.s/ day. This treated water is sent to the top of the aquarium passing through a pipe 8. The water still containing nitric acid enters a denitrification module 9 consisting of high polymer beads 10 in which photosynthetic bacteria are collectively fixed and is now free from nitric acid thanks to the photo synthetic bacteria.