In order to create and maintain a clean, healthy aquatic environment, three stages of filtration are required: mechanical, chemical and biological. Aquarium water is drawn into the canister and through a succession of stacked media trays for full-contact filtration -- virtually no bypass. The water is then returned to your aquarium, sparkling-clean and fish-healthy. And when it’s time to service your canister filter, all you do is push a lever, turn the key, and lift out the easy-access media trays. The Mechanical Filtration Polishing Filter Pads part in this is to screen out fine particulate dirt and debris.