To provide a biologically water-purifying apparatus capable of dealing with every kind of water tank used for appreciating fish and shellfish, for transporting them in a living state, for raising them, for raising deep-sea fish, etc., by optional treatment capacity, and capable of keeping water quality for a long period, without exchanging water.

This biologically water-purifying apparatus is structured so that a microorganism culturing chamber is formed by piling up microorganism culturing units each filled with a filter medium which is implanted with aerobic and anaerobic microorganisms, water of a water tank is circulated by introducing air or oxygen into bottom parts of the culturing units, and the water of the water tank richly containing dissolved oxygen is formed by air reservoirs. Thus, the apparatus is miniaturized and simultaneously treats with the water-purifying treatment capacity optionally required.

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