Iv heard stories .. weather there is truth in them or not but keeping small tanks , which most Betta are kept in as a single fish iv found , are hard work in comparison to a large volume of water , again I don’t know how true it is and have just listened to advice from a breeder / kinda friend and his shop / shed nearly all his tanks under 10gallon has no filter , he reasoning is it’s a waste of filter ... makes little sense to me but he seems to find that the time it takes to change water every 48 hours is far less hassle than actually cycling the filter , maybe personal preference or just crazy I don’t know