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 fish tank water healthy every moment. It contains 15 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 sundries, 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 sundries 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. | Quick Installation: Just remove used cartridge and rinse new cartridge under cool water to remove dust. After rinsing, insert caftridge into flter chamber to protect the health and safety of your fish all the time. | High-quality After-sales Service: HiTauing produces high-quality filter cartridges for fish tank filter, and is in a leading position in the quality and craftsmanship of all our products. We maintain a thorough quality control process to ensure compatibility with TOP FIN PF20, PF30, PF40 and PF75 Power Filters. If you have any questions, please feel free to contact us!