The frequency a filter will require cleaning will depend on many factors, these include, size, efficiency & type of media used in the filter, as well as size, type of fish and the frequency, quantity & type of food used. When you see a decrease in the flow rate this would be a good time to clean the media. Keeping a note of this will enable you to anticipate when the filtration system will next require maintenance. Don’t forget to clean filter media in waste aquarium water to maintain the colony of bacteria.