Fluval Pre-Filter consists of inert, solid ceramic rings used to trap large and medium solid particles before they clog the finer sieves of the media baskets above. The media forces water into many complex paths as it moves through this layer. This further increases its mechanical and biological activity and expands the actual contacted filter area. An additional benefit, it supports large populations of bacteria for proper biological action on ammonia and nitrite. This reduces the need for frequent changes of the active filter media. Pre-Filter is permanent, and requires only occasional rinsing. This media can be used in fresh and marine environments without difficulty.