The present disclosure provides a catalyst filler for purifying water in aquariums and a preparation method and a use thereof. The catalyst filler comprises iron element, carbon element, nickel element and rare earth element； in which the contents of each element in the catalyst filler by mass percent are: iron element 30 wt %-70 wt %； carbon element 20 wt %-50 wt %； nickel element 1 wt %-10 wt %； rare earth element 0.1 wt %-2 wt %. The catalyst filler of the present disclosure, in the use environment, can result in generation of hydroxyl groups and dissociation to micro-element ions, which can degrade nitrites, organic amines and organic sulfides in water bodies, and convert phosphoric acid into precipitates. The organic molecules treated by hydroxyl groups are more easily utilized by denitrifying bacteria, thereby increasing efficiency of denitrification-denitrogenation and purifying the water.
