PURPOSE: To provide a water tank for fishes such as saltwater fishes or aquarium fishes, capable of readily changing water.

CONSTITUTION: In this water tank for fishes, a vessel member 1 is composed of a light-transmitting material and forms an internal space 11 for water storage, and a bottom member 2 is arranged on the side of the bottom 12 of the vessel member 1 and closes the internal space 11 with the bottom 12 side, and a diaphragm member 3 has a number of through holes 31 to 3n on the face thereof and is fitted cross the internal space 11 of the vessel member 1 in a state spaced from the bottom member 2 so as to divide the internal attached on the bottom member 2.