Eheim Substrat Pro Biological Filter Media (Sintered Pearl-Shaped Glass) 450ml Made of highly porous artifiical sintered glass material Unbeatable surface area structure of 1800 sq ft per gallon Can be reused by lightly rinsing with aquarium water every 1 to 3 months and replaced every 3 to 6 months Provides longer intervals between required maintenance Perfect size for Aquaball or Aquastyle filter