The present utility model discloses an aquarium with a fresh water replenishing system comprising an upper tank, a connecting cavity, a lower cabinet and a fresh water replenishing system, with the said upper tank fixed on the lower cabinet, and the said connecting cavity fixed at an inner corner of the upper tank. It effectively utilizes space to store a fresh water replenishing box with a small space occupied. It is provided with a salt water and fresh water circulating system, making it easy to keep salt water and fresh water fish. It is provided with a water baffle to reduce water velocity, reducing noise. All components of its upper tank are provided on the same wall and cover this wall, contributing to good visual effects, safety and aesthetics.