You wont need a really big filter if you go down that route, the smallest will be fine as they don't like a lot of flow anyway. But for stable water conditions then really a filter is the only way to go. And I know you can do daily water changes but I think that is really stressful for the fish, with the water constantly changing. A little advise betta's prefer longer and shallow as they need to breath surface air. (they can actually drown) so best to avoid tall narrow tanks which you see so much off nowadays.