The short answer
Water should pass through your media in this order: mechanical first, biological second, chemical last. In flow terms that’s coarse and fine sponge (traps debris) → ceramic rings or bio-balls (house bacteria) → activated carbon or resin (polishes the water). This sequence protects the delicate biological media from clogging and lets each stage do its job.
Why the sequence matters
Mechanical media — sponges and floss — grab physical waste. Put it first and the grit never reaches your bio media, whose fine pores clog easily. Biological media sits in the middle, bathed in pre-cleaned, oxygen-rich water so the beneficial bacteria thrive. Chemical media goes last, treating water that’s already free of particles, so the carbon works on dissolved compounds rather than getting caked in mud.
Setting up the baskets
In a typical canister, from the inlet outward:
- Coarse sponge — stops leaves and big debris.
- Fine sponge or filter floss — polishes out fine particles.
- Ceramic rings / bio media — the biological heart of the filter.
- Carbon or purigen (optional) — only if you need to remove tannins, medication or odours.
If you’re building a filter for the first time, our how to set up a new aquarium filter walkthrough covers loading and priming step by step.
Don’t disturb the whole lot at once
When you clean, never replace or rinse all your media at the same time. The bio media holds your cycle; strip it all and you risk an ammonia spike. Rinse sponges in old tank water, and swap media gradually. See how to clean an aquarium filter for the safe method, what is aquarium filter media for the basics, and browse the aquarium filters hub.