Maglife Usa AMX70102 250-Pack AMX NitraStrate Polymer Marine Aquarium Filter Substrate Beads, Black NitraStrate filter medium is a special polymer bead made from a particular formula to be almost neutrally buoyant and porous The size and shape of each NitraStrate particle allows the maximum empty space between them Anaerobic colonization occurs inside each NitraStrate particle and remains even when substrate is allowed to dry Almost neutrally buoyant, fish love to swim through it and hunt for food Available in black color; includes 250 beads per pack