EHEIM Prefilter. This pre-filter retains a large part of the coarse dirt, before the water reaches the filter itself (external filter). The pre-filter is installed in place of the suction pipe. Its base is located in the aquarium floor. The water is sucked in through the large filter basket. It is lined with filter material, which is easy to clean and replace. For cleaning simply remove the basket with the plant tongs, for example.