Getting the right cartridge starts with the model number, not the cartridge. Filter ranges reuse names that differ by one digit, and the digit often changes the cartridge size. Look the model up first and the part follows.

Where to look for the model number

Manufacturers do not standardise this, so check in this order:

  • The filter body. Check the housing, the underside or inside of the lid, and the head unit on a canister. Brands differ on whether it is molded into the plastic or printed on a label.
  • The owner’s manual or the box, if you still have either. This is often the same document this site cites. Marineland’s Penguin 200 page, for example, cites a manual that lists MODELS: 100, 150, 200 and 350 and gives the Rite-Size cartridge for each.
  • An old cartridge or its packaging. Some cartridges carry a part number, which can be matched back to the model that uses it.
  • A size marking, on internal filters. Some internal filters are identified by size rather than model number — Aqueon’s QuietFlow E Internal 20 takes the cartridge size its cartridge page calls Medium, for instance.

Two Size 10 filters do not take the same cartridge: the QuietFlow E Internal 10 takes Small, while the QuietFlow LED PRO 10 takes Medium. The number in the name does not fix the part.

Write down the whole name, not part of it

Record the brand and the exact model or size, in the form the manufacturer uses:

Two filters from the same brand can share a number but not a cartridge. Aqueon, for example, publishes one cartridge page covering several filter families, and the size you need depends on which family and size you own.

When the model number is missing

Older or discontinued filters are the hard case. If the body has no marking:

  1. Search the filter’s shape and colour against the manufacturer’s discontinued product photos.
  2. Measure the cartridge slot if you still have a cartridge, and compare it with a cartridge chart. Marineland publishes a dimensions chart for its Rite-Size cartridges.
  3. Check the manual PDF if you can find the filter family, then match the model list in the manual.

If you cannot establish the model, it is better to stop than to guess: buying a cartridge for the wrong model is the failure mode this site exists to avoid.

Then look it up

Once you have the exact model, find it in the reference. Each filter page separates the replacements the manufacturer itself lists from the ones third parties merely claim, because those two things are not the same evidence.