Abstract: A cartridge for biological regeneration of an aquatic medium such as an aquarium is prepared by providing a receptacle containing an organic compost, and having means for allowing passage of water into the receptacle, through the compost and out of the receptacle. The compost contains nitrophilic bacteria of the azobacter family and is produced by complete fermentation of a mixture of animal manures and vegetable wastes. The receptable has two faces and permits passage of water from one face toward the other. A water permeable layer such as a gravel layer is adjacent each of the faces for retaining the compost. One of the faces may include a bottom panel having a plurality of water passage openings and a removable film sealing the openings. The other face may have a removable cover, and a plate having a plurality of water passage openings positioned between the cover and adjacent water permeable layer. The cartridge may be arranged at the bottom of an aquarium or in an external aquarium filter.