Biological media

Biological media is filter material designed with a porous structure that gives beneficial bacteria a large surface area to colonize, since these bacteria live attached to surfaces rather than floating free in the water. Common forms include ceramic rings, sintered glass rings and porous bio-balls, usually placed in a dedicated stage of a canister or hang-on-back filter.

The bacteria living on biological media are what actually carry out the nitrogen cycle inside a running tank, converting ammonia to nitrite and then nitrite to nitrate. Because this colony builds up gradually over weeks, biological media is treated differently from mechanical media like filter floss during cleaning, since rinsing it in tap water or replacing it too often can kill off a significant part of the bacteria colony a tank depends on.

When cleaning a filter, biological media should be rinsed gently in removed tank water rather than under the tap, and swapped out gradually rather than all at once if it ever needs replacing. A common mistake is scrubbing or replacing all the biological media during a routine cleaning, which can trigger a mini ammonia spike similar to an early cycling problem in an otherwise established tank.