Choosing filter media
Mechanical, biological and chemical filter media explained, and which ones are worth using.
Filter media come in three types. Mechanical media catch debris. Biological media give nitrifying bacteria a home. Chemical media remove dissolved waste, color, odor or phosphate. Most tanks do best with a little of each, in that order of flow.
What you’ll need
How to choose
- Start with good biological filtration. Porous media or established rock is the base of every healthy tank. Don’t replace it all at once; rinse it gently in tank water.
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Add one chemical media for clear water. Carbon or an all-in-one like Chemi-Pure is the simplest upgrade. Chemi-Pure lasts about 2–4 months depending on bioload.
Chemi-Pure EliteView product →
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Use phosphate media if you need it. Test first. In reef tanks, PhosGuard or PhosBan helps when phosphate is high. Seachem says to test after 4 days and replace PhosGuard if phosphate hasn’t dropped to around 0.02 mg/L.
Seachem PhosGuardView product →
- Put media where water flows through it. Seachem and Chemi-Pure both recommend a high-flow spot. Chemi-Pure says not to put its bags inside a filter sock.
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Use the right bag. Loose media like Purigen and PhosGuard go in a fine mesh bag.
Seachem PurigenView product →
Good habits
- Rinse new media before use unless the label says otherwise.
- Replace chemical media on schedule; exhausted media stops working without any sign. Purigen turns dark brown or black when it’s done.
- If you run several bags, swap one at a time.
- Hold off on carbon and phosphate media while cycling a new tank.
Where media goes
- Canister filter: mechanical pads first, then biological media, then a bag of chemical media.
- Hang-on-back filter: many, like the Seachem Tidal, have a basket for adding your own media.
- Sump: after the filter socks or skimmer section, in a spot with steady flow, or in a media reactor.
Common mistakes
- Packing media so tight that water goes around it instead of through it.
- Adding a large amount of phosphate media at once to a reef with high phosphate; lower it gradually.
- Throwing out all biological media at once.
Each product’s page has a “How to use” section with the manufacturer’s amounts per tank size.
Quick picks
- One simple upgrade for clear water: Chemi-Pure Elite (reef) or Chemi-Pure Blue.
- Yellow water or high organics: Seachem Purigen.
- Odor and color: Seachem MatrixCarbon.
- High phosphate in a reef: PhosGuard or PhosBan.
- Freshwater ammonia emergency support: Seachem Zeolite.