A filter that's never swapped out doesn't fail cleanly — it just quietly stops doing the three separate jobs it was doing, one at a time, until the tank is only being filtered by whatever's left.

This cartridge is built to do three things: polyester foam on the outside traps fine debris, carbon inside removes pollutants and odours, and zeolite removes ammonia. It's made specifically to fit the Marina i110 and i160 internal filters (filter code A308). The i110 needs one cartridge; the i160 needs two, replaced one at a time rather than both together so the tank doesn't lose its beneficial bacteria all at once.

• Fits Marina i110 and i160 internal filters (code A308)
• Polyester foam traps fine debris
• Carbon removes pollutants and odours
• Zeolite removes ammonia
• i110 needs 1 cartridge; i160 needs 2

Getting the count right. It's easy to buy the wrong quantity for an i160, which needs two cartridges running at once rather than one — and even then, replace them one at a time rather than both together, so the tank isn't left without any established bacteria while a new cartridge settles in.

Who it's for: owners running a Marina i110 or i160 internal filter who need a straight replacement cartridge rather than a full filter unit.