The IceCap 4in Media Cup can hold just about any type of filtration media, from filter floss, ceramic media, carbon or GFO. Just put the Media Cup in place of your 4" filter sock and fill it with your favorite filter media. High-flow slots are cut at the bottom of the media cup ensuring water is forced to travel all the way down to the bottom of the cup and giving all your media proper contact time. We highly recommend containing any loose media in the included mesh media bag. The Filter Media Cup is an ideal and cost-effective replacement to filter socks as well. If all you want is to polish the water of free-floating particles, then placing a few handfuls of a filter floss in the cup and simply replace the dirty filter media every few days. Replaces 4in filter socks. Create your own custom filter tower. Compatible with most pre-made sumps Including IceCap and RedSea. Includes media bag.