The EHEIM SUBSTRATpro is a high-performance biological filter medium made of spherical sintered glass beads designed for both freshwater and marine aquariums. It offers an extremely large surface area for beneficial bacteria colonisation, reducing nitrite peaks and extending maintenance intervals. The material is washable and reusable, allowing you to preserve the bacterial colony rather than replacing media frequently. It is best used in the biological stage of your filtration system, following mechanical and before chemical filtration.