Each Filtral features two different filter sponges, as well as bio-surface elements and filter pebbles which helps to ensure mechanical and biological cleaning in the filter is optimised. These need replacing regularly to ensure continuous capacity of your filter system, ensuring dirt isn't able to build up.
The health of your media will be dependant on the pond and how regularly it is cleaned. If maintained well, and due to the great quality of Oase items, these should be replaced once - twice a year.
Please note that the UVC bulb is not included, and should be replaced yearly. If you're looking for foams for the older style of the Oase Filtral, you can find them here.