On my 5.5 gallon Betta tank I currently have a Top Fin HOB filter with a Fluval pre-filter sponge on it's intake tube to help slow down it's output stream. So far it works marvelously. My question is: Since I am using this pre-filter do I still need to use the filter cartridge that is used inside the HOB filter? Obviously it would save me money if I don't need to keep replacing them. The HOB filter also comes with a plastic bio filter which of course I will keep using. Also I know I need to rinse the pre-filter every water change in tank water and it's reusable.