Aquarium Filter Media Order: What Goes Where

A filter is not a bucket you stuff with media. Water flows through it in one direction, and the order the media sits in decides whether the filter runs well or clogs in a week.

The three jobs, in order

Every filter does three things, and they belong in this sequence:

1. Mechanical removes the visible stuff: fish waste, uneaten food, plant bits. This is sponge, foam or filter floss, and it goes first so debris is caught before it reaches anything else.
2. Biological is the heart of the filter: porous media that houses the bacteria converting toxic ammonia to nitrite to nitrate. This goes second, in clean water, so it never clogs.
3. Chemical is optional polishing: carbon or Purigen that clears color and odor. It goes last, where the water is already clean.

Why the order matters

Put bio or chemical media before the mechanical stage and it clogs with the gunk the sponge should have trapped. A clogged bio block loses the water flow its bacteria need, and clogged carbon stops adsorbing almost immediately. You end up rebuying media that a two-dollar sponge would have protected.

A worked example

The one rule that protects your cycle: never replace all the biological media at once. It carries your bacteria colony, and swapping it wholesale restarts the tank's cycle. Rinse it in old tank water and reuse it.

Pick the media for each stage from the filter media rankings, and size the filter itself with the filter GPH calculator.