Abstract: The present invention relates to a fish tank cleaner having a discharge means for discharging the excrements of fishes together with the water in a fish tank, a filter means for filtering the excrements of fishes in the discharging water, and a returning means for returning the filtered water to the fish tank, the fish tank cleaner including: a discharge port formed below the filter means, for discharging the water staying in a cleaner body to the outside; a viewing window mounted on the top surface of a cover covering the cleaner body, for checking out the interior of the cleaner body; an illuminator mounted on the underside of the cover, for illuminating the interior of the cleaner body; and a sensor mounted on the underside of the cover, for sensing the water not discharged through the filter means.