The next best thing to Filter Socks, the Bio Cup R supplements your traditional Filter Sock making reefing stress free. It is not only reusable, but easy to insert and remove when changing out filter pads, allowing your water to always be crystal clear. It works as a filter media basket that can hold different types of filter media such as filter floss, filter foam and pads, activated carbon, GFO, and bio media such as ESHOPPS Bio Lux. It fits perfectly into any 4”x7” filter sock cradle and easily accommodates stacking of different media types.