Filter media that is in the form of a filter pad should be replaced when the original shape is flattened or is limp. The loft of the material should be similar to how it was originally. If the material is clogging sooner than it used to, it should be replaced. Most biological filter media pads last two seasons because it is not washed often enough to harm the material. Pre-filter media that is used for mechanical filtration wears out faster because it is being washed often. Filter media in the form of plastic tape or bio-balls will last for many years and only needs to be cleaned periodically to keep it fresh.