A water tank filter having a double structure including an inner filter having a small pore size for trapping fine particles of filth from tank water and situated within a hollow outer filter having a large pore size for trapping coarse particles of filth. The inner filter is situated in roughly the upper half of the internal space of the outer filter. Disclosed also is a water tank purifier including the water tank filter. The water tank filter and purifier can effectively remove both coarse and fine particles of filth from a water tank. The filter does not quickly get clogged, and is easy to handle.