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Search online for the owners manual if he no longer has it, then read it.
Then search this forum for "eheim filter cleaning" and see what you turn up. I wrote some info awhile back so seach for Eheim and my name.
Best to get your Dad to show you how to clean it, and to check to see that all parts are in good shape. You may need a new O-ring or impeller or impeller cover. You can get that from a local store, maybe, or online from the better fish places (BigAlsOnline.com, DrsFosterSmith.com, PetSolutions.com and there are others too)