The biOrb filter cartridge works like a waste bin, trapping dirt and keeping it out of the way of your precious fish. After 1-2 months, the cartridge is full and should be replaced with a new one, and a partial water change of approx 30% should be carried out at the same time. This anti-algae filter also contains algicide to help keep the water in your biOrb aquarium clean and algae free, and the kit includes an algae cleaning cloth and a sachet of water preparation chemicals.