An external aquarium filter comprises a flow resistive, porous member disposed between a partition wall and the filter housing. Responsive to stopping the pump, water in the intake chamber begins to reversely flow out of the intake chamber into the aquarium tank through the intake tube due to siphoning, water in the filtering chamber flows back to the intake chamber through the porous member, the reverse flow is faster than water flowing into the intake chamber such that the siphoning breaks when the water level of the intake chamber drops below that of the filtering chamber, the water in the filtering chamber continues to flow back to the intake chamber through the porous member until both the filtering chamber and the intake chamber have the same water level, and sufficient priming water is thus stored in the intake chamber for a future restarting of the filter.