The short answer
Filter media is the material inside your filter that actually cleans the water. It comes in three types — mechanical (traps debris), biological (houses beneficial bacteria) and chemical (absorbs dissolved compounds). Water flows through them in that order: mechanical first, biological second, chemical last. Together they keep the water clear, safe and cycled.
The three types
- Mechanical media — sponges, foam pads and filter floss. They physically strain out fish waste, uneaten food and debris. This is what makes water look clear.
- Biological media — ceramic rings, sintered glass, bio-balls. Their huge, porous surface hosts the bacteria that convert toxic ammonia to nitrite, then to safer nitrate. This is the heart of your filter.
- Chemical media — activated carbon, resins like Purigen, zeolite. They chemically bind dissolved substances: tannins, odours, medication residue. Optional, and best used for a purpose rather than left in permanently.
How the layers work together
Mechanical media goes first so grit is trapped before it clogs the fine pores of the bio media. The biological layer then gets clean, oxygen-rich water — ideal for bacteria. Chemical media sits last, polishing already-clean water. For the full sequence see what order should filter media go in.
Looking after it
Media is mostly reusable — rinse, don’t replace, wherever you can. Read how often should I replace filter media and how to clean an aquarium filter, and understand the biology in what is biological filtration. To choose a filter, browse the aquarium filters hub.