Strong Compatibility: The specific size of our aquarium filter cartridge is 6 in L x 4.5 in W. It is very suitable for Top Fin PF20, PF30, PF40 and PF75 Power Filters. With this aquarium filter cartridge, you can keep your aquatic organisms healthy every moment. It contains 6 pack aquarium filter cottons, which you can change every 2-4 weeks. Excellent Filtration Performance: The filter cotton is made of cotton and carbon and has 3-layer filtering effect, which can filter and absorb 99% of the impurities harmful to fish, so as to keep the water source clean and the healthy growth of fish. Long-term Replacement of the Aquarium Filter: According to the test, the filter cotton has already withstood too much impurities for no more than four weeks, so you need to replace it with a new one to keep the water source healthy and clean at all times. Suggest to change filter cartridge every 2-4 weeks.