API Bio-Chem Stars are a biological filtration media that grows massive colonies of bacteria. They are formed using a patented process that creates a 50 - 70 micron internal pore network. This pore size is ideal for growing beneficial bacteria, including nitrifying bacteria that remove toxic ammonia and nitrite. This pore size also creates optimal oxygen and water transfer through the stars, which keeps the bacteria healthy and active. Use when starting or maintaining an aquarium.