AbstractA breeding aquarium having a filtering chamber therein to allow the space above the filtering chamber to be utilized as a part of the breeding chamber to extend the space of the breeding chamber. The filtering chamber extends throughout the width of the aquarium and water is passed through the filter layer horizontally, so that the filtering efficiency is high for a small size of the filter chamber. The filter layer is a dual layer consisting of the lower layer having a lower resistance to the flow of water and the upper layer having a higher resistance. An auxiliary filter consisting of a net of a synthetic resin, glass wool, or the like is inserted in the circulation path of water to preliminarily filter the water so that the stopping up of the small holes formed in the partition plates defining various chambers is prevented. A reflector plate is provided to convert a part of the circulating water into an ascending current to agitate the water existing above the filter chamber so that the agitated water can better absorb air or oxygen to feed filtering fungi and to increase the chance of the agitated water to contact a biotic membrane of the filtering fungi to more effectively purify water. The filter chamber can be utilized as an aquatic plant culturing chamber by planting the upper layer of the filter layer with aquatic plants still retaining a filtering function as an auxiliary filter chamber. In this case a main filter chamber is provided.