Pretty much any small/mini hang on back filter would be able to do the job (aquaclear, azoo, deep blue). There are some mini canister filters that would be fine as well (tom's rapids, exo-terra, zoomed). I am sure that there are some internal filters that would do the job as well, but I don't have any experience with those. I would focus on finding one that has adjustable flow so that you can keep your betta from being overwhelmed by the current & also can be loaded with whatever filter media you choose vs having to buy only cartridges by whatever manufacturer makes the filter. I am running an exo-terra fx 200 turtle filter on my betta's 5 gallon. I placed the spray bar vertically vs horizontally, facing a corner. This seems to have tamed the flow while still providing good circulation. Hope this helps.