Filter wool. A fully synthetic quality product for freshwater and saltwater. Depending on the type of filter and tank size, it can be used alone or in combination with other hobby filter materials. Rinse well with warm water before first use. The Hobby Aqualon 1000g filter wool is an essential accessory for any aquarist who values a clean and healthy aquarium. With its ability to support bacterial colonization, it not only serves as filter material but also reliably traps coarse dirt. The special feature of this filter wool is that it expands and pops back up after being compressed, ensuring smooth filtration at all times.