An external aquarium filter having a housing which is mountable onto a wall of an aquarium tank. The housing includes an intake compartment and a filter compartment which in turn includes a contaminated water chamber receiving contaminated water drawn into the intake compartment from the aquarium tank and a filter chamber for filtering the water. A motor unit supported at the filter housing has an impeller which sits into it depending from a collar which fits into the intake compartment of the filter. The impeller draws water from the aquarium tank into the intake compartment of the filter. The impeller is encased in a tubular collar. The tubular collar has two opposing windows. The window, which faces the front wall of the intake compartment, is offset in a clockwise direction, as viewed from above, relative to the window which faces the rear wall of the intake compartment, to effectively close off the right edge thereof, to thereby reduce turbulence in the water flowing in the filter.