The Three Jobs a Filter Does

Mechanical, biological and chemical. Every filter does all three, and most problems come from neglecting the middle one.

Mechanical

Sponges and floss trap particles. This is the part you rinse, and the part people mean when they say the filter is “dirty”.

Biological

Ceramic media and mature sponge host the bacteria that process ammonia. This is the part you must not scrub — rinse it gently in tank water, never under the tap.

Chemical

Carbon and resins remove dissolved compounds. Genuinely useful after medication; optional the rest of the time.