Fluval Pre-Filter Ceramic Rings External Filters. Fluval Pre-Filter is designed to capture solid waste particles before they reach other media. This prevents the premature clogging of finer filters and extends their life, reducing the frequency of replacing media. Key Features : Mechanical filter media Effectively traps medium to large solid waste and debris Prevents premature clogging and extends life of subsequent filter media Ideal for medium and coarse filtration Hollow body promotes even water distribution For best results, replace every 6 months Convenient 750 g (26.45 oz) format Ideal for use with most canister filters For use in freshwater and saltwater aquariums.