Every aquarist needs to think about bacteria. Aquarium cycling is when fish tank bacteria grow to consume fish waste and convert ammonia -> nitrite -> nitrate -> nitrogen gas. During and after the aquarium cycle, bacteria need biological media to live on. With proper maintenance and beneficial bacteria, aquarium nitrate levels will remain low. Bio balls and ceramic rings are two basic biofilter choices. Many hobbyists use highly porous ceramic plates or bricks housed in the sump. Bio pellets are plastic media that house and feed bacteria.