Fluval U3 Carbon Cartridge

What usually goes wrong with a small internal filter is not the pump. It is that the media inside it has quietly stopped doing anything, because carbon has a finite capacity and nothing about a full one looks different from an empty one.

This is a two-sided cartridge, which is why it covers two jobs at once. The polyester pad side takes out fine debris and particulate before it circulates again; the carbon side handles dissolved substances, colourants and odours, which Fluval class as the second stage of filtration. It suits freshwater, marine and reptile setups, and the box holds two cartridges.

• Two cartridges per pack
• Polyester pad side for fine debris and contaminants
• Carbon side for dissolved substances, colourants and odours
• For freshwater, marine or reptile environments
• Made for the Fluval U3 Underwater Filter

On when to change it. Fluval publish no replacement interval on anything supplied to us, so nothing here tells you whether a cartridge is a monthly item or a quarterly one — which matters more than usual with carbon, because a loaded cartridge carries on straining water while it has stopped adsorbing anything. Check the manual that came with your filter. And do check the model: this is cut for the U3, and Fluval's other U-series filters take different cartridges.

Who it's for: anyone running a Fluval U3 who wants the correct media rather than a cut-to-fit sponge, and keepers who would rather have a spare in the cupboard than an empty filter.