The foam filter pad replaces the floss and carbon pads. As I think about it the foam is the insert for the Whisper Triad hang on filter. Trim it just a bit and it fits perfect. For a 400 you would need 2, for my old Skilter 250 it only took 1. You can rinse this one out once a week or so and you never have to replace it again, you might alternate rinsing one then the other if you are worried about bacteria, no more high priced floss! You can add a small bag of carbon in the filter housing if you want to run carbon and it works better anyway that way.