An electrically-unplugged aquarium having an automatic water-changing device comprises a water inlet tube, a first container, a second container, a third container and an aquarium. A first curved tube is perpendicularly and insertedly arranged on the bottom portion of the first container. When the water inlet tube inflows water to the first container and the water level of the first container is higher than the top of the first curved tube, a water drainage process is actuated. The second container, which is utilized to contain water drained from the first container, is partitioned into a first space and a second space, in which a second curved tube is perpendicularly and insertedly arranged on the bottom portion of the first space and a third curved tube is perpendicularly and insertedly arranged on the bottom portion of the second space, and the height of the top of the second curved tube is the same as that of the third curved tube. When the water level of the second container is higher than the top ends of the second and third curved tubes, the water drainage process is actuated. The third container is utilized to contain water drained from the first space, and a fourth curved tube is perpendicularly and insertedly arranged on the bottom portion of the third container. When the water level of the third container is higher than the top end of the fourth curved tube, the water drainage process is actuated. An S-type curved tube, which is straddled on one side of the aquarium, has one end that is downwardly extended to the bottom portion of the aquarium and another end that is upwardly straddled with a water drainage curved tube. When the water drained from the second space flows to the aquarium to raise the water level thereof up to the height of the top of the water drainage curved tube, the water drainage process is actuated. When the water level of the aquarium is lowered to the height of the straddled end of the water drainage curved tube, the water drainage p
