Aquarium filter ranges are full of near-identical names: 106 and 107, 306 and 307, 200 and 250, “Size 10” and “Size 20”. The numbers look interchangeable. The cartridges frequently are not.
Generations change the part
A range can keep its naming pattern while the media that fits it changes. The clearest documented case in this reference: Fluval lists one Bio-Foam product for the 06 Series (206, 306) and the 07 Series (207, 307), so a single pack is described as fitting two model numbers at once. You can see the pairing on the Bio-Foam, 2-Pack part page and in the OEM listing each filter page cites.
That pairing does not mean the two filters are the same. It means the manufacturer’s current media is the one it lists for both. Confirm your own model is named on the listing rather than assuming the pairing applies to you.
Numbers are reused across ranges
Different product lines within one brand can use the same small numbers for unrelated things. A “20” in one range may be a tank size; in another it may be a position in the range. Aqueon, for example, publishes one cartridge page whose sizes cover several distinct filter families, and the correct size depends on which family and which number you own.
What to do about it
- Match on the full model name, including the family when there is one.
- Prefer a source that prints a model-to-cartridge mapping over one that infers it from a name.
- When a cartridge is listed for two models, check whether your model is one of them rather than assuming the pair applies to you.
The general rule
The model number is the identifier that matters; the brand is not enough, and the shape of the number tells you nothing. That is why the entries in this reference are keyed to exact models, and why a page for one model will not silently borrow the cartridge of a similarly named neighbour.