Ecofriendly density sponge filter for freshwater tanks Mechanical and biological filtration: the sponge traps large waste particles while bacteria colonize the surface to promote nitrogen cycling, helping maintain clear water. Fish friendly design: filtration occurs without trapping or stressing fish, making it suitable for tanks with active or delicate residents. Versatile filtration role: use as a primary filter, a secondary filter, or attach to an external or filter as a prefiltration unit to improve overall performance. Odor and dirt control: the density material absorbs dirt and speeds up the removal of waste, helping to reduce water odor. Compatible and easy to use: designed to fit most external or filters; simple installation and maintenance. Use scenarios Scenario 1: Set up as the main filtration in a small to midsize freshwater aquarium to keep water clear and healthier for fish. Scenario 2: Add as a prefiltration stage to an external canister or hangon filter to enhance filtration efficiency and extend filter life. Scenario 3: Use in fry or quarantine tanks where gentle, oxygenated filtration protects young or stressed fish while maintaining water quality.