A protein skimmer for filtering and cleaning water in an aquarium having a foam generating assembly having an air chamber (73) and a nozzle (74) in the air chamber, the nozzle being provided with a plurality of air holes (741) and being connected to an inlet tube (71), the inlet tube communicating with a foam cavity within a vertical guide tube (20) for foam, the guide tube (20) generally being tapered as a whole and including an open upper end (202), a bottom end (203) and a hollow cone-shaped circumferential wall being formed between the upper end and the bottom end and encircling the foam cavity (21) thereby tapering toward and communicating with the upper end (202).