A box filter comprises of a casing and at least two filters, which are installed inside the casing and behind the influx channel and have a buffer gap between two adjacent filters; the influx channel, which is installed on the external wall of the box filter. In addition, more than one guide vane is installed at the inlet of the influx channel and the bottom of the guide vane leans against the filter so as to eliminate the clog that forms the channel and so as to stop backflow of the sewage into the aquarium or container of the sift device while the box filter or sift device is removed from the aquarium. The guide vane can effectively improve drawbacks of the box filter of a prior art as well as enhance the filtration effect.