Undergravel filters
 

Are undergravel filters ok to use with Betta fish ?

Unless you can put a reverse flow powerhead on them so the mulm is pushed up through the gravel instead of downward into the area under the plate, UGF's aren't really worth it, and that kind of powerhead would be a bit much for a Betta. A sponge filter or one of those Azoo Mini filters or Whisper in-tank filters would work ok, as long as you do at partial gravel vac every time you do a water change (25-30% once or twice a week).